One document matched: draft-ietf-eai-downgrade-12.xml
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<rfc category="exp" ipr="trust200902" docName="draft-ietf-eai-downgrade-12.txt">
<!-- ipr: full2026 / noDerivativeWorks2026 / none -->
<front>
<title abbrev="UTF8SMTP Downgrade">
Downgrading mechanism for Email Address Internationalization
</title>
<author initials="K.F." surname="Fujiwara" fullname="Kazunori Fujiwara" role="editor">
<organization abbrev="JPRS">Japan Registry Services Co., Ltd.</organization>
<address>
<postal>
<street>Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda</street>
<city>Chiyoda-ku</city>
<region>Tokyo</region>
<code>101-0065</code>
<country>Japan</country>
</postal>
<phone>+81 3 5215 8451</phone>
<email>fujiwara@jprs.co.jp</email>
</address>
</author>
<author initials="Y.Y." surname="YONEYA" fullname="Yoshiro YONEYA" role="editor">
<organization abbrev="JPRS">Japan Registry Services Co., Ltd.</organization>
<address>
<postal>
<street>Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda</street>
<city>Chiyoda-ku</city>
<region>Tokyo</region>
<code>101-0065</code>
<country>Japan</country>
</postal>
<phone>+81 3 5215 8451</phone>
<email>yone@jprs.co.jp</email>
</address>
</author>
<date month="March" day="2" year="2009" />
<area>Applications</area>
<workgroup>Email Address Internationalization (EAI)</workgroup>
<!-- <area ...>, <workgroup ...>, <keyword ...>, <keyword ...> <note..> -->
<abstract>
<t>
Traditional mail systems handle only ASCII characters in SMTP
envelope and mail header fields.
The Email Address Internationalization (UTF8SMTP) extension allows
UTF-8 characters in SMTP envelope and mail header fields.
To avoid rejecting internationalized Email messages when a server
in the delivery path does not support the UTF8SMTP extension,
some sort of converting mechanism is required.
This document describes a downgrading mechanism
for Email Address Internationalization.
Note that this is a way to downgrade, not tunnel. There is no
associated up-conversion mechanism, although internationalized
email clients might use original internationalized addresses or
other data when displaying or replying to downgraded messages.
</t>
</abstract>
</front>
<middle>
<section title="Introduction" anchor="intro">
<t>
Traditional mail systems which are defined
by <xref target="RFC5321" /> and <xref target="RFC5322" />
allow ASCII characters in SMTP envelope and mail header field values.
The UTF8SMTP extension
<xref target="RFC4952" />,
<xref target="RFC5335" /> and
<xref target="RFC5336" />
allows UTF-8 characters in SMTP envelope and mail header field values.
</t>
<t>
If an envelope address or header field contains non-ASCII
characters, the message cannot be delivered unless every system
in the delivery path supports UTF8SMTP.
This document describes a downgrading mechanism to avoid rejection of
such messages when a server which does not support the UTF8SMTP
extension is encountered.
Downgrading mechanism converts envelope and header fields to an
all-ASCII representation.
</t>
<t>
<xref target="RFC5335" />
allows UTF-8 characters to be used in
mail header fields and MIME header fields. The downgrading
mechanism specified here converts mail header fields and MIME
header fields to ASCII.
</t>
<t>
This document does not change any protocols
except by defining new header fields. It describes the conversion
method from the internationalized email envelopes/messages
which are defined in
<xref target="RFC4952" />
<xref target="RFC5335" />
<xref target="RFC5336" />
to the traditional email envelopes/messages
which are defined in
<xref target="RFC5321" />
<xref target="RFC5322" />.
</t>
<t>
<xref target="RFC5336"/> section 2.2 defines when
downgrading occurs.
If the SMTP client has an UTF8SMTP envelope or an internationalized message
and the SMTP server doesn't support the UTF8SMTP SMTP extension,
then the SMTP client MUST NOT send a UTF8SMTP envelope
or an internationalized message to the SMTP server.
The section shows 4 choices.
The fourth choice is downgrading, as described here.
</t>
<t>
Downgrading may be implemented
in MUAs, MSAs, MTAs which act as the SMTP client,
or in MDAs, POP servers, IMAP servers which store or offer
UTF8SMTP envelopes or internationalized messages
to non-UTF8SMTP compliant systems
which include message stores.
</t>
<t>
This document tries to define the downgrading process clearly and
it preserves the original information as much as possible.
</t>
<t>
Downgrading in UTF8SMTP consists of the following four parts:
<list style="symbols">
<t>New header fields definition</t>
<t>SMTP downgrading</t>
<t>Email header fields downgrading</t>
<t>MIME header fields downgrading</t>
</list>
<vspace blankLines="1" />
</t>
<t>
In <xref target="downgradedheader" />,
many header fields starting with "Downgraded-"
are introduced.
They preserve the original envelope information and the original header fields.
</t>
<t>
The SMTP downgrading is described in <xref target="smtpdowngrading" />.
It generates ASCII only envelope information from an UTF8SMTP envelope.
</t>
<t>
The Email header fields downgrading is described in <xref target="HeaderDowngrading" />.
It generates ASCII only header fields.
</t>
<t>
The MIME header fields are expanded in <xref target="RFC5335" />.
The MIME header fields downgrading is described in <xref target="MIMEheaderdowngrading" />.
It generates ASCII only MIME header fields.
</t>
<t>
Displaying downgraded messages
which originally contain internationalized E-mail addresses
or internationalized header fields
is described in an another document (<xref target="I-D.ietf-eai-downgraded-display" />).
</t>
</section>
<section title="Terminology">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in RFC
2119 <xref target="RFC2119" />.
</t>
<t>
All specialized terms used in this specification are defined in the
EAI overview <xref target="RFC4952" /> or in
<xref target="RFC5321" /><xref target="RFC5322" />,
MIME documents
<xref target="RFC2045" />
<xref target="RFC2047" />
<xref target="RFC2183" />
<xref target="RFC2231" />.
The terms "ASCII address", "internationalized email address", "non-ASCII
address", "i18mail address", "UTF8SMTP", "message" and "mailing list"
are used with the definitions from <xref target="RFC4952" /> document.
</t>
<t>
This document depends on <xref target="RFC5335" />,
<xref target="RFC5336" />, and
<xref target="RFC5337" />.
Key words used in these document are used in this document, too.
</t>
<t>
The term "non-ASCII" is an UTF-8 string which contains at least
one non-ASCII character.
</t>
<t>
An "UTF8SMTP envelope" has Email originator/recipient addresses
expanded by <xref target="RFC5336"/> and
<xref target="RFC5337"/>.
</t>
<t>
An "UTF8SMTP message" is Email messages expanded by
<xref target="RFC5335" />.
</t>
</section>
<section title="New header fields definition" anchor="downgradedheader">
<t>
New header fields starting with "Downgraded-" are defined here to
preserve those original envelope and header field values which contain
UTF-8 characters. During downgrading, one new "Downgraded-"
header field is added for each original envelope or header field
which cannot be passed as-is to a server which does not support
UTF8SMTP. The original envelope or header field is removed or
rewritten. Only those envelope and header fields which contain
non-ASCII characters are affected. The result of this process
is a message which is compliant with existing email
specifications <xref target="RFC5321" /> and <xref target="RFC5322" />.
The original internationalized information can be retrieved by examining the
"Downgraded-" header fields which were added.
</t>
<section title="Envelope information preservation header fields">
<t>
SMTP envelope downgraded information <downgraded-envelope-addr>
consists of
the original non-ASCII address and the downgraded all-ASCII address.
<figure>
<artwork><![CDATA[
downgraded-envelope-addr = [FWS] "<" [ A-d-l ":" ] uMailbox
FWS "<" Mailbox ">" ">" [CFWS]
]]></artwork>
</figure>
<uMailbox> is defined in <xref target="RFC5336" />;
<Mailbox> and <A-d-l> are defined in <xref target="RFC5321" />, section 4.1.2. </t>
<t>
Two header fields "Downgraded-Mail-From:" and "Downgraded-Rcpt-To:" are defined
to preserve SMTP envelope downgraded information.
The header field syntax is specified as follows:
<figure>
<artwork><![CDATA[
fields =/ downgradedmailfrom / downgradedrcptto
downgradedmailfrom = "Downgraded-Mail-From:" unstructured CRLF
downgradedrcptto = "Downgraded-Rcpt-To:" unstructured CRLF
]]></artwork>
</figure>
The unstructured content is downgraded-envelope-addr treated as if it were
unstructured with <xref target="RFC2047" /> encoding (and charset UTF-8) as needed.
</t>
</section>
<section title="Address header field preservation header fields" anchor="AddressHeaderPreservationHeaders">
<t>
The address header fields preservation header fields are defined
to preserve the original header field.
Their value field holds the original header field value.
The header field syntax is specified as follows:
<figure>
<artwork>
<![CDATA[
fields =/ known-downgraded-headers ":" unstructured CRLF
known-downgraded-headers = "Downgraded-" original-headers
original-headers = "From" / "Sender" /
"To" / "Cc" / "Bcc" /
"Reply-To" /
"Resent-From" / "Resent-Sender" /
"Resent-To" / "Resent-Cc" / "Resent-Bcc" /
"Resent-Reply-To" /
"Return-Path" /
"Disposition-Notification-To"
]]>
</artwork>
</figure>
Preserving a header field in a downgraded header field is defined as:
<list style="numbers">
<t>Generate new downgraded header field whose value is the original header field value.</t>
<t>
Treat the generated header field content as if it were unstructured,
and then apply <xref target="RFC2047" /> encoding with charset UTF-8 as necessary so
the result is ASCII.
</t>
</list>
</t>
</section>
<section title="Unknown header fields preservation header fields" anchor="unknownencapsulation">
<t>
The unknown header fields preservation header fields are defined to
encapsulate those original header fields which contain non-ASCII
characters and are not otherwise provided for in the this
specification. The encapsulation header field name is the
concatenation of "Downgraded-" and the original name. The value
field holds the original header field value.
</t>
<t>
The header field syntax is specified as follows:
<figure>
<artwork>
<![CDATA[
fields =/ unknown-downgraded-headers ":" unstructured CRLF
unknown-downgraded-headers = "Downgraded-" original-header-field-name
original-header-field-name = field-name
field-name = 1*ftext
ftext = %d33-57 / ; Any character except
%d59-126 ; controls, SP, and
; ":".
]]>
</artwork>
</figure>
</t>
<t>
Encapsulating a header field in a "Downgraded-" header field is defined as:
<list style="numbers">
<t>Generate new "Downgraded-" header field whose value is the original header field value.</t>
<t>
Treat the generated header field content as if it were unstructured,
and then apply <xref target="RFC2047" /> encoding with charset UTF-8
as necessary so the result is ASCII.
</t>
<t>Remove the original header field.</t>
</list>
</t>
</section>
</section>
<section title="SMTP Downgrading" anchor="smtpdowngrading">
<t>
Target of downgrading elements in SMTP envelope are below:
</t>
<t>
<list style="symbols">
<t><reverse-path> of MAIL FROM command</t>
<t><forward-path> of RCPT TO command</t>
<t>ORCPT parameter of RCPT TO command</t>
</list>
</t>
<section title="Path element downgrading" anchor="pathdowngrading">
<t>
Downgrading the <path> of MAIL FROM and RCPT TO commands uses ALT-ADDRESS parameter defined in
<xref target="RFC5336"/>. A SMTP command is downgradable if
the <path> contains non-ASCII address and the command has an ALT-ADDRESS parameter which specifies an ASCII address.
Since only non-ASCII addresses are downgradable, specifying an
ALT-ADDRESS value for an all-ASCII address is invalid for use
with this specification, and no interpretation is assigned to
it. This restriction allows for future extension of the
specification even though no such extensions are currently
anticipated.
</t>
<t>
Note that even if no downgrading is performed on the envelope,
message header fields and message body MIME header fields
that contain non-ASCII characters MUST be downgraded.
This is described in <xref target="HeaderDowngrading" /> and <xref target="MIMEheaderdowngrading" />.
</t>
<t>
When downgrading, replace each <path> which contains non-ASCII mail address
with its specified alternative ASCII address
and preserve the original information using "Downgraded-Mail-From"
and "Downgraded-Rcpt-To" header fields as defined in <xref target="downgradedheader" />.
Before replacing, decode the ALT-ADDRESS parameter
value because it is encoded as xtext <xref target="RFC3461" />.
</t>
<t>
To avoid disclosing recipient addresses,
the downgrading process MUST NOT add "Downgraded-Rcpt-To:" header field
if the SMTP downgrading targets multiple recipients.
See <xref target="security" /> for more detail.
</t>
<t>
As a result of the recipient address downgrading,
the domain part of the recipient address prior to downgrading
might be different from the domain part of the new recipient address.
If the result of address resolution
for the domain part of the new recipient address contains
the server at the connection destination of the SMTP session for the
recipient address prior to downgrading,
the SMTP connection is valid for the new recipient address.
Otherwise, the downgrading process MUST NOT send the downgraded message
to the new recipient address via the connection
and MUST try to send the downgraded message to the new recipient address.
</t>
</section>
<section title="ORCPT downgrading" anchor="orcptdowngrading">
<t>
The "RCPT TO" command can have an ORCPT parameter if the DSN
extension <xref target="RFC3461" /> is supported.
If the ORCPT parameter contains an "utf-8" type address
and the address contains raw non-ASCII characters,
the address MUST be converted to utf-8-addr-xtext form.
Those forms are described in <xref target="RFC5337"/> and
clarified by successor documents such as
<xref target="I-D.ietf-eai-dsnbis" />.
</t>
<t>
Before converting to utf-8-addr-xtext form, remove xtext encoding.
</t>
</section>
</section>
<section title="Email header fields downgrading" anchor="HeaderDowngrading">
<t>
This section defines the conversion method to ASCII for each header field
which may contain non-ASCII characters.
</t>
<t>
<xref target="RFC5335" /> expands
Received: header fields,
<xref target="RFC5322" /> ABNF elements
<mailbox>, <word>, <comment>, <unstructured>,
<xref target="RFC2045" /> ABNF element <value>.
</t>
<section title="Downgrading method for each ABNF element" anchor="DowngradeElements">
<t>
Header field downgrading is defined below for each ABNF element.
Downgrading an unknown header field is also defined as ENCAPSULATION downgrading.
Converting the header field terminates when no non-ASCII characters
remain in the header field.
</t>
<section title="RECEIVED downgrading" anchor="element:received"><t>
If the header field name is "Received:" and
the FOR clause contains a non-ASCII addresses,
remove the FOR clause from the header field.
Other parts (not counting <comment>s) should not contain non-ASCII values.
</t></section>
<section title="UNSTRUCTURED downgrading" anchor="element:unstructured"><t>
If the header field has an <unstructured> field which contains non-ASCII characters,
apply <xref target="RFC2047" /> encoding with charset UTF-8.
</t></section>
<section title="WORD downgrading" anchor="element:word"><t>
If the header field has any <word>
fields which contains non-ASCII characters,
apply <xref target="RFC2047" /> encoding with charset UTF-8.
</t></section>
<section title="COMMENT downgrading" anchor="element:comment"><t>
If the header field has any <comment>
fields which contains non-ASCII characters,
apply <xref target="RFC2047" /> encoding with charset UTF-8.
</t></section>
<section title="MIME-VALUE downgrading" anchor="element:mimevalue"><t>
If the header field has any <value> elements defined by <xref target="RFC2045" />
and the elements contain non-ASCII characters,
encode the <value> elements
by <xref target="RFC2231" /> with charset UTF-8 and the Language information empty.
If the <value> element is <quoted-string>
and it contains <CFWS> outside the DQUOTE,
remove the <CFWS> before this conversion.
</t></section>
<section title="DISPLAY-NAME downgrading" anchor="element:display-name"><t>
If the header field has any <address> (<mailbox> and <group>) elements and
they have <display-name> elements which contain non-ASCII characters,
encode the <display-name> elements
according to <xref target="RFC2047" /> with charset UTF-8.
DISPLAY-NAME downgrading is the same algorithm as WORD downgrading.
</t></section>
<section title="MAILBOX downgrading" anchor="element:mailbox"><t>
The <mailbox> elements have no equivalent format for non-ASCII addresses.
If the header field has any <mailbox> elements which contain non-ASCII characters,
preserve the header field in each Address header field preservation header field defined in <xref target="AddressHeaderPreservationHeaders" />,
and rewrite each <mailbox> element to ASCII only format.
The <mailbox> element which contains non-ASCII characters is one of three formats.
<vspace blankLines="1" />
<list style="symbols">
<t>[ Display-name ] "<" Utf8-addr-spec 1*FCS "<" Addr-spec ">>"
<vspace blankLines="1" />
Rewrite it as
<vspace blankLines="1" />
[ Display-name ] "<" Addr-spec ">"
<vspace blankLines="1" />
</t>
<t>[ Display-name ] "<" Utf8-addr-spec ">"</t>
<t>Utf8-addr-spec
<vspace blankLines="1" />
Rewrite both as
<vspace />
[ Display-name ] "Internationalized Address " Encoded-word
<vspace blankLines="0" />
" Removed:;"
<vspace blankLines="1" />
where the <Encoded-word> is the original <Utf8-addr-spec> encoded
according to <xref target="RFC2047" />.
</t>
</list>
</t></section>
<section title="ENCAPSULATION downgrading"><t>
if the header field contains non-ASCII characters
and for which no rule is given above,
encapsulate it in a Downgraded header field
described in <xref target="unknownencapsulation" />
as a last resort.
<vspace blankLines="1" />
Applying this procedure to "Received" header field is prohibited.
</t></section>
<section title="TYPED-ADDRESS downgrading"><t>
If the header field contains <utf-8-type-addr>
and the <utf-8-type-addr> contains raw non-ASCII characters,
it is utf-8-address form and
convert it to utf-8-addr-xtext form
as described in <xref target="orcptdowngrading" />.
COMMENT downgrading is also performed in this case.
If the address type is unrecognized and the header field contains non-ASCII characters,
then fall back to using ENCAPSULATION downgrading on the entire header field.
</t></section>
</section>
<section title="Downgrading method for each header field" anchor="DowngradeEachHeader">
<t>
Header fields are listed in <xref target="RFC4021" />.
This section describes the downgrading method for each header field.
</t>
<t>
If the whole mail header field does not contain non-ASCII characters,
email header field downgrading is not required.
Each header field's downgrading method is described below.
</t>
<section title="Address header fields which contain <address>s" anchor="header:address"><t>
<list style="hanging">
<t hangText="From:" />
<t hangText="Sender:" />
<t hangText="To:" />
<t hangText="Cc:" />
<t hangText="Bcc:" />
<t hangText="Reply-To:" />
<t hangText="Resent-From:" />
<t hangText="Resent-Sender:" />
<t hangText="Resent-To:" />
<t hangText="Resent-Cc:" />
<t hangText="Resent-Bcc:" />
<t hangText="Resent-Reply-To:" />
<t hangText="Return-Path:" />
<t hangText="Disposition-Notification-To:" />
</list>
<vspace blankLines="1" />
If the header field contains <mailbox> elements
which contains non-ASCII addresses,
preserve the header field in a downgraded header field before the conversion.
Then perform COMMENT downgrading, DISPLAY-NAME downgrading and MAILBOX downgrading.
</t></section>
<section title="Address header fields with typed addresses" anchor="header:typedaddress"><t>
<list style="hanging">
<t hangText="Original-Recipient:" />
<t hangText="Final-Recipient:" />
</list>
<vspace blankLines="1" />
If the header field contains non-ASCII characters, perform TYPED-ADDRESS downgrading.
</t></section>
<section title="Downgrading Non-ASCII in comments" anchor="header:comments"><t>
<list style="hanging">
<t hangText="Date:" />
<t hangText="Message-ID:" />
<t hangText="Resent-Message-ID:" />
<t hangText="In-Reply-To:" />
<t hangText="References:" />
<t hangText="Resent-Date:" />
<t hangText="Resent-Message-ID:" />
<t hangText="MIME-Version:" />
<t hangText="Content-ID:" />
<t hangText="Content-Transfer-Encoding:" />
<t hangText="Content-Language:" />
<t hangText="Accept-Language:" />
<t hangText="Auto-Submitted:" />
</list>
<vspace blankLines="1" />
These header fields do not contain non-ASCII characters except in comments.
If the header field contains UTF-8 characters in comments,
perform COMMENT downgrading.
</t></section>
<section title="Received header field" anchor="header:received"><t>
<list style="hanging">
<t hangText="Received:" />
</list>
<vspace blankLines="1" />
perform COMMENT downgrading and RECEIVED downgrading.
</t></section>
<section title="MIME Content header fields" anchor="header:mime"><t>
<list style="hanging">
<t hangText="Content-Type:" />
<t hangText="Content-Disposition:" />
</list>
<vspace blankLines="1" />
Perform MIME-VALUE downgrading and COMMENT downgrading.
</t></section>
<section title="Non-ASCII in <unstructured>" anchor="header:unstructured"><t>
<list style="hanging">
<t hangText="Subject:" />
<t hangText="Comments:" />
<t hangText="Content-Description:" />
</list>
<vspace blankLines="1" />
Perform UNSTRUCTURED downgrading.
</t></section>
<section title="Non-ASCII in <phrase>" anchor="header:phrase"><t>
<list style="hanging">
<t hangText="Keywords:" />
</list>
<vspace blankLines="1" />
Perform WORD downgrading.
</t></section>
<section title="Other header fields" anchor="header:undefined"><t>
All other header fields which contains non-ASCII characters
are user-defined, missing from this draft
or future defined header fields.
Perform ENCAPSULATION downgrading.
<vspace blankLines="1" />
If the software understands the header field's structure and a downgrading
algorithm other than ENCAPSULATION is applicable, that software SHOULD use
that algorithm; ENCAPSULATION downgrading is used as a last resort.
<vspace blankLines="1" />
Mailing list header fields (those that start in "List-")
are part of this category.
</t></section>
</section>
</section>
<section title="MIME body part header fields downgrading" anchor="MIMEheaderdowngrading">
<t>
MIME body part header fields may contain non-ASCII characters
<xref target="RFC5335" />.
This section defines the conversion method to ASCII only header fields
for each MIME header field which contains non-ASCII characters.
Parse the message body's MIME structure for all levels and
check each MIME header field whether it contains non-ASCII characters.
If the header field contains non-ASCII characters in the header field value,
the header field is a target of the MIME body part header fields downgrading.
Each MIME header field's downgrading method is described below.
COMMENT downgrading, MIME-VALUE downgrading, UNSTRUCTURED downgrading
are described in <xref target="HeaderDowngrading" />.
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Content-ID:">
<vspace />
The Content-ID: header field does not contain non-ASCII characters except in comments.
If the header field contains UTF-8 characters in comments,
perform COMMENT downgrading.
<vspace blankLines="1" />
</t>
<t hangText="Content-Type:" />
<t hangText="Content-Disposition:">
<vspace />
Perform MIME-VALUE downgrading and
COMMENT downgrading.
<vspace blankLines="1" />
</t>
<t hangText="Content-Description:">
<vspace />
Perform UNSTRUCTURED downgrading.
<vspace blankLines="1" />
</t>
</list>
</t>
</section>
<section title="Security considerations" anchor="security">
<t>
A Downgraded message's header fields contain ASCII characters only.
But they still contain MIME encapsulated header fields which contains
non-ASCII UTF-8 characters.
Furthermore, the body part may contain UTF-8 characters.
Implementations parsing Internet messages need
to accept UTF-8 body parts and UTF-8 header fields which are MIME encoded.
Thus it inherits the security considerations of MIME encoded header fields <xref target="RFC2047" /> and <xref target="RFC3629" />.
</t>
<t>
Rewriting header fields increases the opportunities for
undetected spoofing by the malicious senders.
However rewritten header fields are preserved into Downgraded-*
header fields and parsing Downgraded-* header fields enables
detecting spoofing caused by downgrading.
</t>
<t>
Addresses that do not appear in the message header fields may appear in the
RCPT commands to an SMTP server for a number of reasons. Copying
information from the Envelope into header fields risks inadvertent information
disclosure (see <xref target="RFC5321" /> and
<xref target="smtpdowngrading" />).
Mitigating inadvertent information disclosure is discussed in same place.
</t>
<t>
The techniques described here invalidates methods that depend
on digital signatures over the envelope or any part of the message
which includes the top-level header fields or body part header fields.
Depending on the specific message being downgraded, DKIM
especially, but also possibly S/MIME, PGP, and similar
techniques are all likely to break.
The two obvious mitigations are to stick to 7-bit transport when using these techniques
(as most/all of them presently require),
or make sure you have UTF8SMTP end-to-end when needed.
</t>
<t>
Many gateways and servers on the Internet will
discard header fields with which they are not familiar.
To the extent to which the downgrade procedures depend on
new header fields (e.g., "Downgraded-") to avoid information
loss, the risk of having those header fields dropped and
its implications must be identified. In particular,
if the Downgraded header fields are dropped,
there is no possibility of
reconstructing the original information
at any point (before, during, or after delivery).
Such gateways violate <xref target="RFC2979" /> and can be upgraded to correct the problem.
</t>
<t>
Even though the information is not lost, the original
message cannot be perfectly reconstructed
because some downgrading methods remove information
(see <xref target="element:received" /> and
<xref target="element:mimevalue" />).
Hence, downgrading is a one-way process.
</t>
<t>
While information in any email header field should usually treated with
some suspicion, current email systems commonly employ various
mechanisms and protocols to make the information more trustworthy.
Currently, information in the new Downgraded-* header fields is usually
not inspected by these mechanisms, and may be even less trustworthy
than the traditional header fields.
Note that the Downgraded-* header fields could have been inserted with malicious intent.
(and with content unrelated to the traditional header fields).
</t>
<t>
If an internationalized MUA would simply try to "upgrade" the message for
display purposes (that is, display the information in the
Downgraded-* header fields instead of the traditional header fields), the
effectiveness of the deployed mechanisms and protocols is likely to
be reduced, and the user may be exposed to additional risks.
More guidance on how to display downgraded messages will be given
in <xref target="I-D.ietf-eai-downgraded-display" />.
</t>
<t>
Concerns about the trustworthiness of the Downgraded-* header fields
are not limited to displaying and replying in MUAs, and should be carefully
considered before using them for other purposes as well.
</t>
<t>
See "Security considerations" section in <xref target="RFC4952"/> for more discussion.
</t>
</section>
<section title="Implementation notes">
<section title="RFC 2047 encoding">
<t>
While <xref target="RFC2047" /> has a specific algorithm to deal with whitespace in
adjacent encoded-words, there are a number of deployed implementations
that fail to implement the algorithm correctly. As a result, whitespace
behavior is somewhat unpredictable in practice when multiple encoded words
are used. While RFC 5322 states that implementations SHOULD limit lines
to not more than 78 characters, implementations MAY choose to allow
overlong encoded words in order to work around faulty <xref target="RFC2047" />
implementations. Implementations that choose to do so SHOULD have an
optional mechanism to limit line length to 78 characters.
</t>
</section>
<section title="Trivial downgrading">
<t>
Downgrading is an alternative to avoid the rejection of messages
which require UTF8SMTP support by a server which does not provide
this. Implementing the full specification of this document is
desirable, but a partial implementation is also possible.
</t>
<t>
If a partial downgrading implementation confronts an unsupported
downgrading target, the implementation MUST NOT send the message
to a server which does not support UTF8SMTP. Instead, it MUST
reject the message or generate a notification of
non-deliverability.
</t>
<t>
A partial downgrading, Trivial downgrading is discussed.
It does not support non-ASCII addresses
in SMTP envelope and address header fields,
unknown header fields downgrading,
the MIME body part header fields downgrading.
It supports
<list style="symbols">
<t>some simple header fields downgrading: Subject</t>
<t>comments and display name downgrading: From, To, Cc</t>
<t>trace header field downgrading: Received</t>
</list>
</t>
<t>
Otherwise, the downgrading fails.
</t>
<t>
Trivial downgrading targets mail messages which are generated by
UTF8SMTP aware MUAs and contain
non-ASCII characters in comments, display names, unstructured parts
without using non-ASCII E-mail addresses.
This mail message does not contain non-ASCII E-mail addresses in the SMTP Envelope
and its header fields. But it is not deliverable via a UTF8SMTP un-aware SMTP server.
Implementing full specification downgrading may be hard,
but trivial downgrading saves mail messages without using non-ASCII
addresses.
</t>
</section>
<section title="7bit transport consideration">
<t>
The SMTP client may encounter a SMTP server which does not support the 8BITMIME SMTP extension <xref target="RFC1652" />.
The server does not support "8bit" or "binary" data.
Implementers need to consider converting "8bit" data to "base64" or "quoted-printable" encoded form and adjust the "Content-Transfer-Encoding" header field accordingly.
If the body contains multiple MIME parts, this conversion MUST be performed for each MIME part.
</t>
</section>
</section>
<section title="IANA Considerations" anchor="iana">
<t>
IANA is requested to register the following header fields in the
Permanent Message Header Field Repository, in accordance with the
procedures set out in <xref target="RFC3864" />.
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Mail-From</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Rcpt-To</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-From</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Sender</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-To</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Cc</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Bcc</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Reply-To</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Resent-From</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Resent-Sender</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Resent-To</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Resent-Cc</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Resent-Bcc</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Resent-Reply-To</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Return-Path</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
<list style="hanging">
<t hangText="Header field name:">Downgraded-Disposition-Notification-To</t>
<t hangText="Applicable protocol:">mail</t>
<t hangText="Status:">experimental</t>
<t hangText="Author/change controller:">IETF</t>
<t hangText="Specification document(s):">This document (<xref target="downgradedheader" />)</t>
</list>
<vspace blankLines="1" />
</t>
<t>
Furthermore, IANA is requested to refuse registration of all the field names
that start with "Downgraded-" for unknown header fields downgrading
described in <xref target="unknownencapsulation" />
to avoid conflicts with existing IETF activity (Email Address Internationalization).
</t>
</section>
<section title="Acknowledgements">
<t>
Significant comments and suggestions were received from John
Klensin, Harald Alvestrand, Chris Newman, Randall Gellens,
Charles Lindsey,
Marcos Sanz, Alexey Melnikov, Frank Ellermann, Edward Lewis,
S. Moonesamy and JET members.
</t>
</section>
<section title="Change History">
<t>
This section is used for tracking the update of this document. Will be
removed after finalize.
</t>
<section title="draft-yoneya-ima-downgrade: Version 00">
<t>
<list style="symbols">
<t>Initial version</t>
<t>Followed draft-yeh-ima-utf8headers-00 and
draft-yao-smtpext-00</t>
</list>
</t>
</section>
<section title="draft-yoneya-ima-downgrade: Version 01">
<t>
<list style="symbols">
<t>Document structure was changed</t>
<t>Followed draft-yeh-ima-utf8headers-01 and
draft-yao-smtpext-02</t>
<t>Downgrading requirements were added</t>
<t>SMTP DATA encapsulation method was proposed</t>
<t>Downgrading examples was provided</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 00">
<t>
<list style="symbols">
<t>Followed draft-yeh-ima-utf8headers-01 and
draft-ietf-eai-smtpext-00</t>
<t>No header field downgrading method was proposed</t>
<t>Header encapsulation method was proposed</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 01">
<t>
<list style="symbols">
<t>Followed draft-ietf-eai-utf8headers-00</t>
<t>Header conversion and encapsulation method was merged</t>
<t>Header conversion method was defined in detail</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 02">
<t>
<list style="symbols">
<t>Followed draft-ietf-eai-utf8headers-01 and
draft-ietf-eai-smtpext-01</t>
<t>Specification about algorithmic generated address is removed</t>
<t>No header field downgrading method was removed</t>
<t>SMTP DATA encapsulation method was removed</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 03">
<t>
<list style="symbols">
<t>Followed draft-ietf-eai-utf8headers-03 and
draft-ietf-eai-smtpext-03</t>
<t>Downgraded: and Envelope-Downgraded: headers definition was added</t>
<t>Mail header fields downgrading method was refined</t>
<t>Examples in Appendix A were refined</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 04">
<t>
<list style="symbols">
<t>Followed draft-ietf-eai-utf8headers-06,
draft-ietf-eai-smtpext-07
and draft-ietf-eai-dsn-02</t>
<t>Downgrading requirements and conditions were moved to Introduction.</t>
<t>Descriptions about upgrading were removed.</t>
<t>SPF and DKIM discussion were removed.</t>
<t>Added many header fields downgrading.</t>
<t>Allow address literal rewriting without alternate ASCII address in header fields.</t>
<t>Added MIME body part headers downgrading.</t>
<t>Added ORCPT downgrading.</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 05">
<t>
<list style="symbols">
<t>fixed examples
<list style="symbols">
<t>ALT-ADDRESS parameter mistake</t>
<t>RFC2047(x) notation was changed to encoded-word format</t>
</list>
</t>
<t>Added implementation consideration section and trivial downgrading</t>
<t>Downgraded: and Envelope-Downgraded: headers are separated for each original headers.</t>
<t>Removed list-* header fields downgrading</t>
<t>Changed the way of writing the header field downgrading section</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 06">
<t>
<list style="symbols">
<t>Moved decoding downgraded messages as a separate document</t>
<t>Added a text to UNSTRUCTURED downgrading</t>
<t>Added "replacing SMTP connection" if necessary to SMTP downgrading.</t>
<t>fixed examples</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 07">
<t>
<list style="symbols">
<t>Fixed some typos</t>
<t>Added a text about 7bit transport</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 08">
<t>
<list style="symbols">
<t>Comments from the working group last call (wording)</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 09">
<t>
<list style="symbols">
<t>References</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 10">
<t>
<list style="symbols">
<t>Comments from AD Review</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 11">
<t>
<list style="symbols">
<t>IETF Last call: Comments from Gen-ART and IANA</t>
<t>Added new downgraded header field definitions for Resent-Reply-To, Recent-Bcc and Disposition-Notification-To</t>
<t>Separated "Email header fields downgrading" section into subsections</t>
<t>Updated ORCPT and TYPED-ADDRESS downgrading</t>
</list>
</t>
</section>
<section title="draft-ietf-eai-downgrade: Version 12">
<t>
<list style="symbols">
<t>Comments from IESG</t>
<t>rewrite all 'header' to 'header field'.</t>
</list>
</t>
</section>
</section>
</middle>
<back>
<references title="Normative References">
&rfc4952;
&rfc5335;
&rfc5336;
&rfc5337;
&rfc1652;
&rfc2045;
&rfc2047;
&rfc2183;
&rfc2231;
&rfc5321;
&rfc5322;
&rfc3461;
&rfc3629;
&rfc3864;
&rfc2979;
<!-- &rfc3987; -->
&rfc4021;
&rfc2119;
</references>
<references title="Informative References">
<!-- &I-D.ietf-eai-dsnbis; does not work well. -->
<reference anchor='I-D.ietf-eai-dsnbis'><front>
<title>Internationalized Delivery Status and Disposition Notifications</title>
<author initials='C' surname='Newman' fullname='Chris Newman'>
<organization /></author>
<author initials='A' surname='Melnikov' fullname='Alexey Melnikov'>
<organization /></author>
<date month='December' day='16' year='2008' />
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-eai-dsnbis-00' />
</reference>
<reference anchor='I-D.ietf-eai-downgraded-display'>
<front>
<title>Displaying Downgraded Messages for Email Address Internationalization</title>
<!-- &I-D.ietf-eai-downgraded-display; does not work well. -->
<author initials='K' surname='Fujiwara' fullname='Kazunori Fujiwara'><organization /></author>
<date month='October' day='16' year='2008' />
<abstract><t>This document describes how to display downgraded messages which originally contain internationalized E-mail addresses or internationalized header fields.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-eai-downgraded-display-00' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-ietf-eai-downgraded-display-00.txt' />
</reference>
</references>
<section title="Examples">
<section title="Downgrading example 1">
<t>
This section shows an SMTP Downgrading example.
Consider a mail message where:
<list style="symbols">
<t>
The sender address is "NON-ASCII-local@example.com" which is a non-ASCII
address. Its ASCII alternative is "ASCII-local@example.com" and its display-name is "DISPLAY-local".
</t>
<t>
The "To:" address is "NON-ASCII-remote1@example.net" which is a non-ASCII address. Its
ASCII alternative is "ASCII-remote1@example.net" and its display-name is "DISPLAY-remote1".
</t>
<t>
The "Cc:" address is a non-ASCII address "NON-ASCII-remote2@example.org"
without alternative ASCII address.
Its display-name is "DISPLAY-remote2".
</t>
<t>Three display-names contain non-ASCII characters.</t>
<t>
The Subject header field is "NON-ASCII-SUBJECT" which contains non-ASCII
characters.
</t>
<t>Assuming the "To:" recipient's MTA (example.net) does not support UTF8SMTP.</t>
<t>assuming the "Cc:" recipient's MTA (example.org) supports UTF8SMTP.</t>
</list>
The example SMTP envelope/message is shown in <xref target="example-originaleaismtpsession" />.
In this example, the "To:" recipient's session is the focus.
</t>
<t>
<figure title="Original envelope/message (example 1)" anchor="example-originaleaismtpsession">
<artwork>
<![CDATA[
MAIL FROM: <NON-ASCII-local@example.com>
ALT-ADDRESS=ASCII-local@example.com
RCPT TO: <NON-ASCII-remote1@example.net>
ALT-ADDRESS=ASCII-remote1@example.net
RCPT TO: <NON-ASCII-remote2@example.org>
-------------------------------------------------------------
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
Date: DATE
MAIL_BODY
]]>
</artwork>
</figure>
</t>
<t>
In this example, there are two SMTP recipients, one is "To:", the other is "Cc:".
The SMTP downgrading treats To: session downgrading.
<xref target="example-smtpdowngrading" /> shows SMTP downgraded example.
</t>
<t>
<figure title="SMTP Downgraded envelope/message (example 1)" anchor="example-smtpdowngrading">
<artwork>
<![CDATA[
MAIL FROM: <ASCII-local@example.com>
RCPT TO: <ASCII-remote1@example.net>
-------------------------------------------------------------
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
=?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
=?UTF-8?Q?<ASCII-remote1@example.net>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
Date: DATE
MAIL_BODY
]]>
</artwork>
</figure>
</t>
<t>
After SMTP downgrading, header fields downgrading is performed.
Final downgraded message is shown in <xref target="example-headerconversiondowngrading" />.
Return-Path header field will be added by the final destination MTA.
<figure title="Downgraded message (example 1)" anchor="example-headerconversiondowngrading">
<artwork>
<![CDATA[
Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
=?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
=?UTF-8?Q?<ASCII-remote1@example.net>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
=?UTF-8?Q?<ASCII-local@example.com>>?=
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
=?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Cc: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address
=?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:;
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
=?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Date: DATE
MAIL_BODY
]]>
</artwork>
</figure>
</t>
</section>
<section title="Downgrading example 2">
<t>
In many cases, the sender wants to use non-ASCII address and
the recipient is a traditional mail user.
The SMTP server handing mail for the recipient and/or the recipient's MUA does not support UTF8SMTP extension.
Consider a mail message where:
<list style="symbols">
<t>
The sender address is "NON-ASCII-local@example.com" which is a non-ASCII
address. Its ASCII alternative is "ASCII-local@example.com". It has a display-name "DISPLAY-local" which contains non-ASCII characters.
</t>
<t>
The "To:" address is "ASCII-remote1@example.net" which is ASCII only. It has a display-name "DISPLAY-remote1" which contains non-ASCII characters.
</t>
<t>
The "Subject:" header field is "NON-ASCII-SUBJECT" which contains non-ASCII
characters.
</t>
</list>
The second example envelope/message is shown in
<xref target="example-originaleaismtpsession2" />.
</t>
<t>
<figure title="Original message (example 2)" anchor="example-originaleaismtpsession2">
<artwork>
<![CDATA[
MAIL From: <NON-ASCII-local@example.com>
ALT-ADDRESS=ASCII-local@example.com
RCPT TO: <ASCII-remote1@example.net>
-------------------------------------------------------------
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <ASCII-remote1@example.net>
Date: DATE
MAIL_BODY
]]>
</artwork>
</figure>
</t>
<t>
In this example, SMTP session is downgradable.
<xref target="example-smtpdowngrading2" /> shows SMTP downgraded envelope/message.
</t>
<t>
<figure title="SMTP Downgraded envelope/message (example 2)" anchor="example-smtpdowngrading2">
<artwork>
<![CDATA[
MAIL From: <ASCII-local@example.com>
RCPT TO: <ASCII-remote1@example.net>
-------------------------------------------------------------
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
?=UTF8?Q?<ASCII-local@example.com>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <ASCII-remote1@example.net>
Date: DATE
MAIL_BODY
]]>
</artwork>
</figure>
</t>
<t>
After SMTP downgrading, header fields downgrading is performed.
The downgraded example is shown in <xref target="example-headerconversiondowngrading2" />.
<figure title="Downgraded message (example 2)" anchor="example-headerconversiondowngrading2">
<artwork>
<![CDATA[
Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
=?UTF8?Q?<ASCII-local@example.com>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
=?UTF-8?Q?<ASCII-local@example.com>>?=
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Date: DATE
MAIL_BODY
]]>
</artwork>
</figure>
</t>
</section>
</section>
</back>
</rfc>
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