Internet Engineering Task Force (IETF) J. Klensin
Request for Comments: 6152
STD: 71 N. Freed
Obsoletes: 1652 Oracle
Category: Standards Track M. Rose
ISSN: 2070-1721 Dover Beach Consulting, Inc.
D. Crocker, Ed.
March 2011 SMTP Service Extension for 8-bit MIME Transport
This memo defines an extension to the SMTP service whereby an SMTP
content body consisting of text containing octets outside of the
US-ASCII octet range (hex 00-7F) may be relayed using SMTP.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Although SMTP is widely and robustly deployed, various extensions
have been requested by parts of the Internet community. In
particular, a significant portion of the Internet community wishes to
exchange messages in which the content body consists of a MIME
message [RFC2045][RFC2046][RFC5322] containing arbitrary octet-
aligned material. This memo uses the mechanism described in the SMTP
specification [RFC5321] to define an extension to the SMTP service
whereby such contents may be exchanged. Note that this extension
does NOT eliminate the possibility of an SMTP server limiting line
length; servers are free to implement this extension but nevertheless
set a line length limit no lower than 1000 octets. Given that this
restriction still applies, this extension does NOT provide a means
for transferring unencoded binary via SMTP.
2. Framework for the 8-bit MIME Transport Extension
The 8-bit MIME transport extension is laid out as follows:
1. the name of the SMTP service extension defined here is
2. the EHLO keyword value associated with the extension is 8BITMIME;
3. no parameter is used with the 8BITMIME EHLO keyword;
4. one optional parameter using the keyword BODY is added to the
MAIL command. The value associated with this parameter is a
keyword indicating whether a 7-bit message (in strict compliance
with [RFC5321]) or a MIME message (in strict compliance with
[RFC2046] and [RFC2045]) with arbitrary octet content is being
sent. The syntax of the value is as follows, using the ABNF
notation of [RFC5234]:
body-value = "7BIT" / "8BITMIME"
5. no additional SMTP verbs are defined by this extension; and
6. the next section specifies how support for the extension affects
the behavior of a server and client SMTP.
3. The 8bit-MIMEtransport Service Extension
When a client SMTP wishes to submit (using the MAIL command) a
content body consisting of a MIME message containing arbitrary lines
of octet-aligned material, it first issues the EHLO command to the
server SMTP. If the server SMTP responds with code 250 to the EHLO
command, and the response includes the EHLO keyword value 8BITMIME,
then the server SMTP is indicating that it supports the extended MAIL
command and will accept MIME messages containing arbitrary octet-
The extended MAIL command is issued by a client SMTP when it wishes
to transmit a content body consisting of a MIME message containing
arbitrary lines of octet-aligned material. The syntax for this
command is identical to the MAIL command in RFC 5321, except that a
BODY parameter must appear after the address. Only one BODY
parameter may be used in a single MAIL command.
The complete syntax of this extended command is defined in RFC 5321.
The esmtp-keyword is BODY, and the syntax for esmtp-value is given by
the syntax for body-value shown above.
The value associated with the BODY parameter indicates whether the
content body that will be passed using the DATA command consists of a
MIME message containing some arbitrary octet-aligned material
("8BITMIME") or is encoded entirely in accordance with RFC 5321
A server that supports the 8-bit MIME transport service extension
shall preserve all bits in each octet passed using the DATA command.
Naturally, the usual SMTP data-stuffing algorithm applies, so that a
content that contains the five-character sequence of
<CR> <LF> <DOT> <CR> <LF>
or a content that begins with the three-character sequence of
<DOT> <CR> <LF>
does not prematurely terminate the transfer of the content. Further,
it should be noted that the CR-LF pair immediately preceding the
final dot is considered part of the content. Finally, although the
content body contains arbitrary lines of octet-aligned material, the
length of each line (number of octets between two CR-LF pairs) is
still subject to SMTP server line length restrictions (which can
allow as few as 1000 octets, inclusive of the CR-LF pair, on a single
line). This restriction means that this extension provides the
necessary facilities for transferring a MIME object with the 8BIT
content-transfer-encoding, it DOES NOT provide a means of
transferring an object with the BINARY content-transfer-encoding.
Once a server SMTP supporting the 8bit-MIMEtransport service
extension accepts a content body containing octets with the high-
order (8th) bit set, the server SMTP must deliver or relay the
content in such a way as to preserve all bits in each octet.
If a server SMTP does not support the 8-bit MIME transport extension
(either by not responding with code 250 to the EHLO command, or by
not including the EHLO keyword value 8BITMIME in its response), then
the client SMTP must not, under any circumstances, attempt to
transfer a content that contains characters outside of the US-ASCII
octet range (hex 00-7F).
A client SMTP has two options in this case: first, it may implement a
gateway transformation to convert the message into valid 7-bit MIME,
or second, it may treat the barrier to 8-bit as a permanent error and
handle it in the usual manner for delivery failures. The specifics
of the transformation from 8-bit MIME to 7-bit MIME are not described
by this RFC; the conversion is nevertheless constrained in the
1. it must cause no loss of information; MIME transport encodings
must be employed as needed to insure this is the case, and
2. the resulting message must be valid 7-bit MIME.
4. Usage Example
The following dialogue illustrates the use of the 8bit-MIMEtransport
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250-dbc.mtview.ca.us says hello
S: 250 8BITMIME
C: MAIL FROM:<email@example.com> BODY=8BITMIME
S: 250 <firstname.lastname@example.org>... Sender and 8BITMIME ok
C: RCPT TO:<email@example.com>
S: 250 <firstname.lastname@example.org>... Recipient ok
S: 354 Send 8BITMIME message, ending in CRLF.CRLF.
S: 250 OK
S: 250 Goodbye
5. Security Considerations
This RFC does not discuss security issues and is not believed to
raise any security issues not already endemic in electronic mail and
present in fully conforming implementations of RFC 5321, including
attacks facilitated by the presence of an option negotiation
mechanism. Since MIME semantics are transport-neutral, the 8BITMIME
option provides no more added capability to disseminate malware than
is provided by unextended 7-bit SMTP.
6. IANA Considerations
6.1. SMTP Service Extension Registration
This document defines an SMTP and Submit service extension. IANA has
updated the 8BITMIME entry in the SMTP Service Extensions registry,
Description: SMTP and Submit transport of 8-bit MIME content
Parameters: See Section 2 in this specification.
E. Stefferud was an original author. This version of the
specification was produced by the YAM working group.
Original acknowledgements: This document represents a synthesis of
the ideas of many people and reactions to the ideas and proposals
of others. Randall Atkinson, Craig Everhart, Risto Kankkunen, and
Greg Vaudreuil contributed ideas and text sufficient to be
considered co-authors. Other important suggestions, text, or
encouragement came from Harald Alvestrand, Jim Conklin,
Mark Crispin, Frank da Cruz, Olafur Gudmundsson, Per Hedeland,
Christian Huitma, Neil Katin, Eliot Lear, Harold A. Miller,
Keith Moore, Dan Oscarsson, Julian Onions, Neil Rickert,
John Wagner, Rayan Zachariassen, and the contributions of the
entire IETF SMTP Working Group. Of course, none of the
individuals are necessarily responsible for the combination of
ideas represented here. Indeed, in some cases, the response to a
particular criticism was to accept the problem identification but
to include an entirely different solution from the one originally
8. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
John C. Klensin
1770 Massachusetts Ave, Ste. 322
Cambridge, MA 02140
Phone: +1 617 245 1457
800 Royal Oaks
Monrovia, CA 91016-6347
Dover Beach Consulting, Inc.
Sacramento, CA 95865-5268
Phone: +1 916 538 2535
D. Crocker (editor)
675 Spruce Dr.
Phone: +1 408 246 8253