actual developers.
The problems associated with ill-formed messages were exacerbated by
the introduction of the split-UA mail reading protocols [3, 26, 5,
21]. These protocols have encouraged the use of SMTP as a posting
protocol, and SMTP servers as relay systems for these client hosts
(which are often only intermittently connected to the Internet).
Historically, many of those client machines lacked some of the
mechanisms and information assumed by SMTP (and indeed, by the mail
format protocol [7]). Some could not keep adequate track of time;
others had no concept of time zones; still others could not identify
their own names or addresses; and, of course, none could satisfy the
assumptions that underlay RFC822's conception of authenticated
addresses.
In response to these weak SMTP clients, many SMTP systems now
complete messages that are delivered to them in incomplete or
incorrect form. This strategy is generally considered appropriate
when the server can identify or authenticate the client, and there
are prior agreements between them. By contrast, there is at best
great concern about fixes applied by a relay or delivery SMTP server
that has little or no knowledge of the user or client machine.
The following changes to a message being processed MAY be applied
when necessary by an originating SMTP server, or one used as the
target of SMTP as an initial posting protocol:
- Addition of a message-id field when none appears
- Addition of a date, time or time zone when none appears
- Correction of addresses to proper FQDN format
The less information the server has about the client, the less likely
these changes are to be correct and the more caution and conservatism
should be applied when considering whether or not to perform fixes
and how. These changes MUST NOT be applied by an SMTP server that
provides an intermediate relay function.
In all cases, properly-operating clients supplying correct
information are preferred to corrections by the SMTP server. In all
cases, documentation of actions performed by the servers (in trace
fields and/or header comments) is strongly encouraged.
7. Security Considerations
7.1 Mail Security and Spoofing
SMTP mail is inherently insecure in that it is feasible for even
fairly casual users to negotiate directly with receiving and relaying
SMTP servers and create messages that will trick a naive recipient
into believing that they came from somewhere else. Constructing such
a message so that the "spoofed" behavior cannot be detected by an
expert is somewhat more difficult, but not sufficiently so as to be a
deterrent to someone who is determined and knowledgeable.
Consequently, as knowledge of Internet mail increases, so does the
knowledge that SMTP mail inherently cannot be authenticated, or
integrity checks provided, at the transport level. Real mail
security lies only in end-to-end methods involving the message
bodies, such as those which use digital signatures (see [14] and,
e.g., PGP [4] or S/MIME [31]).
Various protocol extensions and configuration options that provide
authentication at the transport level (e.g., from an SMTP client to
an SMTP server) improve somewhat on the traditional situation
described above. However, unless they are accompanied by careful
handoffs of responsibility in a carefully-designed trust environment,
they remain inherently weaker than end-to-end mechanisms which use
digitally signed messages rather than depending on the integrity of
the transport system.
Efforts to make it more difficult for users to set envelope return
path and header "From" fields to point to valid addresses other than
their own are largely misguided: they frustrate legitimate
applications in which mail is sent by one user on behalf of another
or in which error (or normal) replies should be directed to a special
address. (Systems that provide convenient ways for users to alter
these fields on a per-message basis should attempt to establish a
primary and permanent mailbox address for the user so that Sender
fields within the message data can be generated sensibly.)
This specification does not further address the authentication issues
associated with SMTP other than to advocate that useful functionality
not be disabled in the hope of providing some small margin of
protection against an ignorant user who is trying to fake mail.
7.2 "Blind" Copies
Addresses that do not appear in the message headers may appear in the
RCPT commands to an SMTP server for a number of reasons. The two
most common involve the use of a mailing address as a "list exploder"
(a single address that resolves into multiple addresses) and the
appearance of "blind copies". Especially when more than one RCPT
command is present, and in order to avoid defeating some of the
purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
the full set of RCPT command arguments into the headers, either as
part of trace headers or as informational or private-extension
headers. Since this rule is often violated in practice, and cannot
be enforced, sending SMTP systems that are aware of "bcc" use MAY
find it helpful to send each blind copy as a separate message
transaction containing only a single RCPT command.
There is no inherent relationship between either "reverse" (from
MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
transaction ("envelope") and the addresses in the headers. Receiving
systems SHOULD NOT attempt to deduce such relationships and use them
to alter the headers of the message for delivery. The popular
"Apparently-to" header is a violation of this principle as well as a
common source of unintended information disclosure and SHOULD NOT be
used.
7.3 VRFY, EXPN, and Security
As discussed in section 3.5, individual sites may want to disable
either or both of VRFY or EXPN for security reasons. As a corollary
to the above, implementations that permit this MUST NOT appear to
have verified addresses that are not, in fact, verified. If a site
disables these commands for security reasons, the SMTP server MUST
return a 252 response, rather than a code that could be confused with
successful or unsuccessful verification.
Returning a 250 reply code with the address listed in the VRFY
command after having checked it only for syntax violates this rule.
Of course, an implementation that "supports" VRFY by always returning
550 whether or not the address is valid is equally not in
conformance.
Within the last few years, the contents of mailing lists have become
popular as an address information source for so-called "spammers."
The use of EXPN to "harvest" addresses has increased as list
administrators have installed protections against inappropriate uses
of the lists themselves. Implementations SHOULD still provide
support for EXPN, but sites SHOULD carefully evaluate the tradeoffs.
As authentication mechanisms are introduced into SMTP, some sites may
choose to make EXPN available only to authenticated requestors.
7.4 Information Disclosure in Announcements
There has been an ongoing debate about the tradeoffs between the
debugging advantages of announcing server type and version (and,
sometimes, even server domain name) in the greeting response or in
response to the HELP command and the disadvantages of exposing
information that might be useful in a potential hostile attack. The
utility of the debugging information is beyond doubt. Those who
argue for making it available point out that it is far better to
actually secure an SMTP server rather than hope that trying to
conceal known vulnerabilities by hiding the server's precise identity
will provide more protection. Sites are encouraged to evaluate the
tradeoff with that issue in mind; implementations are strongly
encouraged to minimally provide for making type and version
information available in some way to other network hosts.
7.5 Information Disclosure in Trace Fields
In some circumstances, such as when mail originates from within a LAN
whose hosts are not directly on the public Internet, trace
("Received") fields produced in conformance with this specification
may disclose host names and similar information that would not
normally be available. This ordinarily does not pose a problem, but
sites with special concerns about name disclosure should be aware of
it. Also, the optional FOR clause should be supplied with caution or
not at all when multiple recipients are involved lest it
inadvertently disclose the identities of "blind copy" recipients to
others.
7.6 Information Disclosure in Message Forwarding
As discussed in section 3.4, use of the 251 or 551 reply codes to
identify the replacement address associated with a mailbox may
inadvertently disclose sensitive information. Sites that are
concerned about those issues should ensure that they select and
configure servers appropriately.
7.7 Scope of Operation of SMTP Servers
It is a well-established principle that an SMTP server may refuse to
accept mail for any operational or technical reason that makes sense
to the site providing the server. However, cooperation among sites
and installations makes the Internet possible. If sites take
excessive advantage of the right to reject traffic, the ubiquity of
email availability (one of the strengths of the Internet) will be
threatened; considerable care should be taken and balance maintained
if a site decides to be selective about the traffic it will accept
and process.
In recent years, use of the relay function through arbitrary sites
has been used as part of hostile efforts to hide the actual origins
of mail. Some sites have decided to limit the use of the relay
function to known or identifiable sources, and implementations SHOULD
provide the capability to perform this type of filtering. When mail
is rejected for these or other policy reasons, a 550 code SHOULD be
used in response to EHLO, MAIL, or RCPT as appropriate.
8. IANA Considerations
IANA will maintain three registries in support of this specification.
The first consists of SMTP service extensions with the associated
keywords, and, as needed, parameters and verbs. As specified in
section 2.2.2, no entry may be made in this registry that starts in
an "X". Entries may be made only for service extensions (and
associated keywords, parameters, or verbs) that are defined in
standards-track or experimental RFCs specifically approved by the
IESG for this purpose.
The second registry consists of "tags" that identify forms of domain
literals other than those for IPv4 addresses (specified in RFC821
and in this document) and IPv6 addresses (specified in this
document). Additional literal types require standardization before
being used; none are anticipated at this time.
The third, established by RFC821 and renewed by this specification,
is a registry of link and protocol identifiers to be used with the
"via" and "with" subclauses of the time stamp ("Received: header")
described in section 4.4. Link and protocol identifiers in addition
to those specified in this document may be registered only by
standardization or by way of an RFC-documented, IESG-approved,
Experimental protocol extension.
9. References
[1] American National Standards Institute (formerly United States of
America Standards Institute), X3.4, 1968, "USA Code for
Information Interchange". ANSI X3.4-1968 has been replaced by
newer versions with slight modifications, but the 1968 version
remains definitive for the Internet.
[2] Braden, R., "Requirements for Internet hosts - application and
support", STD 3, RFC1123, October 1989.
[3] Butler, M., Chase, D., Goldberger, J., Postel, J. and J.
Reynolds, "Post Office Protocol - version 2", RFC937, February
1985.
[4] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
Message Format", RFC2440, November 1998.
[5] Crispin, M., "Interactive Mail Access Protocol - Version 2", RFC
1176, August 1990.
[6] Crispin, M., "Internet Message Access Protocol - Version 4", RFC
2060, December 1996.
[7] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC822, August 1982.
[8] Crocker, D. and P. Overell, Eds., "Augmented BNF for Syntax
Specifications: ABNF", RFC2234, November 1997.
[9] De Winter, J., "SMTP Service Extension for Remote Message Queue
Starting", RFC1985, August 1996.
[10] Fajman, R., "An Extensible Message Format for Message
Disposition Notifications", RFC2298, March 1998.
[11] Freed, N, "Behavior of and Requirements for Internet Firewalls",
RFC2979, October 2000.
[12] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC2045, December 1996.
[13] Freed, N., "SMTP Service Extension for Command Pipelining", RFC
2920, September 2000.
[14] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security
Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
RFC1847, October 1995.
[15] Gellens, R. and J. Klensin, "Message Submission", RFC2476,
December 1998.
[16] Kille, S., "Mapping between X.400 and RFC822/MIME", RFC2156,
January 1998.
[17] Hinden, R and S. Deering, Eds. "IP Version 6 Addressing
Architecture", RFC2373, July 1998.
[18] Klensin, J., Freed, N. and K. Moore, "SMTP Service Extension for
Message Size Declaration", STD 10, RFC1870, November 1995.
[19] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
"SMTP Service Extensions", STD 10, RFC1869, November 1995.
[20] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
"SMTP Service Extension for 8bit-MIMEtransport", RFC1652, July
1994.
[21] Lambert, M., "PCMAIL: A distributed mail system for personal
computers", RFC1056, July 1988.
[22] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC1035, November 1987.
Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC1034, November 1987.
[23] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC2047,
December 1996.
[24] Moore, K., "SMTP Service Extension for Delivery Status
Notifications", RFC1891, January 1996.
[25] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC1894, January 1996.
[26] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
53, RFC1939, May 1996.
[27] Partridge, C., "Mail routing and the domain system", RFC974,
January 1986.
[28] Partridge, C., "Duplicate messages and SMTP", RFC1047, February
1988.
[29] Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", STD 7, RFC793, September 1981.
[30] Postel, J., "Simple Mail Transfer Protocol", RFC821, August
1982.
[31] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[32] Resnick, P., Ed., "Internet Message Format", RFC2822, April
2001.
[33] Vaudreuil, G., "SMTP Service Extensions for Transmission of
Large and Binary MIME Messages", RFC1830, August 1995.
[34] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC1893,
January 1996.
10. Editor's Address
John C. Klensin
AT&T Laboratories
99 Bedford St
Boston, MA 02111 USA
Phone: 617-574-3076
EMail: klensin@research.att.com
11. Acknowledgments
Many people worked long and hard on the many iterations of this
document. There was wide-ranging debate in the IETF DRUMS Working
Group, both on its mailing list and in face to face discussions,
about many technical issues and the role of a revised standard for
Internet mail transport, and many contributors helped form the
wording in this specification. The hundreds of participants in the
many discussions since RFC821 was produced are too numerous to
mention, but they all helped this document become what it is.
APPENDICES
A. TCP Transport Service
The TCP connection supports the transmission of 8-bit bytes. The
SMTP data is 7-bit ASCII characters. Each character is transmitted
as an 8-bit byte with the high-order bit cleared to zero. Service
extensions may modify this rule to permit transmission of full 8-bit
data bytes as part of the message body, but not in SMTP commands or
responses.
B. Generating SMTP Commands from RFC822 Headers
Some systems use RFC822 headers (only) in a mail submission
protocol, or otherwise generate SMTP commands from RFC822 headers
when such a message is handed to an MTA from a UA. While the MTA-UA
protocol is a private matter, not covered by any Internet Standard,
there are problems with this approach. For example, there have been
repeated problems with proper handling of "bcc" copies and
redistribution lists when information that conceptually belongs to a
mail envelopes is not separated early in processing from header
information (and kept separate).
It is recommended that the UA provide its initial ("submission
client") MTA with an envelope separate from the message itself.
However, if the envelope is not supplied, SMTP commands SHOULD be
generated as follows:
1. Each recipient address from a TO, CC, or BCC header field SHOULD
be copied to a RCPT command (generating multiple message copies if
that is required for queuing or delivery). This includes any
addresses listed in a RFC822 "group". Any BCC fields SHOULD then
be removed from the headers. Once this process is completed, the
remaining headers SHOULD be checked to verify that at least one
To:, Cc:, or Bcc: header remains. If none do, then a bcc: header
with no additional information SHOULD be inserted as specified in
[32].
2. The return address in the MAIL command SHOULD, if possible, be
derived from the system's identity for the submitting (local)
user, and the "From:" header field otherwise. If there is a
system identity available, it SHOULD also be copied to the Sender
header field if it is different from the address in the From
header field. (Any Sender field that was already there SHOULD be
removed.) Systems may provide a way for submitters to override
the envelope return address, but may want to restrict its use to
privileged users. This will not prevent mail forgery, but may
lessen its incidence; see section 7.1.
When an MTA is being used in this way, it bears responsibility for
ensuring that the message being transmitted is valid. The mechanisms
for checking that validity, and for handling (or returning) messages
that are not valid at the time of arrival, are part of the MUA-MTA
interface and not covered by this specification.
A submission protocol based on Standard RFC822 information alone
MUST NOT be used to gateway a message from a foreign (non-SMTP) mail
system into an SMTP environment. Additional information to construct
an envelope must come from some source in the other environment,
whether supplemental headers or the foreign system's envelope.
Attempts to gateway messages using only their header "to" and "cc"
fields have repeatedly caused mail loops and other behavior adverse
to the proper functioning of the Internet mail environment. These
problems have been especially common when the message originates from
an Internet mailing list and is distributed into the foreign
environment using envelope information. When these messages are then
processed by a header-only remailer, loops back to the Internet
environment (and the mailing list) are almost inevitable.
C. Source Routes
Historically, the <reverse-path> was a reverse source routing list of
hosts and a source mailbox. The first host in the <reverse-path>
SHOULD be the host sending the MAIL command. Similarly, the
<forward-path> may be a source routing lists of hosts and a
destination mailbox. However, in general, the <forward-path> SHOULD
contain only a mailbox and domain name, relying on the domain name
system to supply routing information if required. The use of source
routes is deprecated; while servers MUST be prepared to receive and
handle them as discussed in section 3.3 and F.2, clients SHOULD NOT
transmit them and this section was included only to provide context.
For relay purposes, the forward-path may be a source route of the
form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST BE fully-
qualified domain names. This form is used to emphasize the
distinction between an address and a route. The mailbox is an
absolute address, and the route is information about how to get
there. The two concepts should not be confused.
If source routes are used, RFC821 and the text below should be
consulted for the mechanisms for constructing and updating the
forward- and reverse-paths.
The SMTP server transforms the command arguments by moving its own
identifier (its domain name or that of any domain for which it is
acting as a mail exchanger), if it appears, from the forward-path to
the beginning of the reverse-path.
Notice that the forward-path and reverse-path appear in the SMTP
commands and replies, but not necessarily in the message. That is,
there is no need for these paths and especially this syntax to appear
in the "To:" , "From:", "CC:", etc. fields of the message header.
Conversely, SMTP servers MUST NOT derive final message delivery
information from message header fields.
When the list of hosts is present, it is a "reverse" source route and
indicates that the mail was relayed through each host on the list
(the first host in the list was the most recent relay). This list is
used as a source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list, it MUST
use its name as known in the transport environment to which it is
relaying the mail rather than that of the transport environment from
which the mail came (if they are different).
D. Scenarios
This section presents complete scenarios of several types of SMTP
sessions. In the examples, "C:" indicates what is said by the SMTP
client, and "S:" indicates what is said by the SMTP server.
D.1 A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host bar.com, to Jones,
Green, and Brown at host foo.com. Here we assume that host bar.com
contacts host foo.com directly. The mail is accepted for Jones and
Brown. Green does not have a mailbox at host foo.com.
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250 HELP
C: MAIL FROM:<Smith@bar.com>
S: 250 OK
C: RCPT TO:<Jones@foo.com>
S: 250 OK
C: RCPT TO:<Green@foo.com>
S: 550 No such user here
C: RCPT TO:<Brown@foo.com>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Blah blah blah...
C: ...etc. etc. etc.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
D.2 Aborted SMTP Transaction Scenario
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250 HELP
C: MAIL FROM:<Smith@bar.com>
S: 250 OK
C: RCPT TO:<Jones@foo.com>
S: 250 OK
C: RCPT TO:<Green@foo.com>
S: 550 No such user here
C: RSET
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
D.3 Relayed Mail Scenario
Step 1 -- Source Host to Relay Host
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250 HELP
C: MAIL FROM:<JQP@bar.com>
S: 250 OK
C: RCPT TO:<@foo.com:Jones@XYZ.COM>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Date: Thu, 21 May 1998 05:33:29 -0700
C: From: John Q. Public <JQP@bar.com>
C: Subject: The Next Meeting of the Board
C: To: Jones@xyz.com
C:
C: Bill:
C: The next meeting of the board of directors will be
C: on Tuesday.
C: John.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
Step 2 -- Relay Host to Destination Host
S: 220 xyz.com Simple Mail Transfer Service Ready
C: EHLO foo.com
S: 250 xyz.com is on the air
C: MAIL FROM:<@foo.com:JQP@bar.com>
S: 250 OK
C: RCPT TO:<Jones@XYZ.COM>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Received: from bar.com by foo.com ; Thu, 21 May 1998
C: 05:33:29 -0700
C: Date: Thu, 21 May 1998 05:33:22 -0700
C: From: John Q. Public <JQP@bar.com>
C: Subject: The Next Meeting of the Board
C: To: Jones@xyz.com
C:
C: Bill:
C: The next meeting of the board of directors will be
C: on Tuesday.
C: John.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
D.4 Verifying and Sending Scenario
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250-VRFY
S: 250 HELP
C: VRFY Crispin
S: 250 Mark Crispin <Admin.MRC@foo.com>
C: SEND FROM:<EAK@bar.com>
S: 250 OK
C: RCPT TO:<Admin.MRC@foo.com>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Blah blah blah...
C: ...etc. etc. etc.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
E. Other Gateway Issues
In general, gateways between the Internet and other mail systems
SHOULD attempt to preserve any layering semantics across the
boundaries between the two mail systems involved. Gateway-
translation approaches that attempt to take shortcuts by mapping,
(such as envelope information from one system to the message headers
or body of another) have generally proven to be inadequate in
important ways. Systems translating between environments that do not
support both envelopes and headers and Internet mail must be written
with the understanding that some information loss is almost
inevitable.
F. Deprecated Features of RFC821
A few features of RFC821 have proven to be problematic and SHOULD
NOT be used in Internet mail.
F.1 TURN
This command, described in RFC821, raises important security issues
since, in the absence of strong authentication of the host requesting
that the client and server switch roles, it can easily be used to
divert mail from its correct destination. Its use is deprecated;
SMTP systems SHOULD NOT use it unless the server can authenticate the
client.
F.2 Source Routing
RFC821 utilized the concept of explicit source routing to get mail
from one host to another via a series of relays. The requirement to
utilize source routes in regular mail traffic was eliminated by the
introduction of the domain name system "MX" record and the last
significant justification for them was eliminated by the
introduction, in RFC1123, of a clear requirement that addresses
following an "@" must all be fully-qualified domain names.
Consequently, the only remaining justifications for the use of source
routes are support for very old SMTP clients or MUAs and in mail
system debugging. They can, however, still be useful in the latter
circumstance and for routing mail around serious, but temporary,
problems such as problems with the relevant DNS records.
SMTP servers MUST continue to accept source route syntax as specified
in the main body of this document and in RFC1123. They MAY, if
necessary, ignore the routes and utilize only the target domain in
the address. If they do utilize the source route, the message MUST
be sent to the first domain shown in the address. In particular, a
server MUST NOT guess at shortcuts within the source route.
Clients SHOULD NOT utilize explicit source routing except under
unusual circumstances, such as debugging or potentially relaying
around firewall or mail system configuration errors.
F.3 HELO
As discussed in sections 3.1 and 4.1.1, EHLO is strongly preferred to
HELO when the server will accept the former. Servers must continue
to accept and process HELO in order to support older clients.
F.4 #-literals
RFC821 provided for specifying an Internet address as a decimal
integer host number prefixed by a pound sign, "#". In practice, that
form has been obsolete since the introduction of TCP/IP. It is
deprecated and MUST NOT be used.
F.5 Dates and Years
When dates are inserted into messages by SMTP clients or servers
(e.g., in trace fields), four-digit years MUST BE used. Two-digit
years are deprecated; three-digit years were never permitted in the
Internet mail system.
F.6 Sending versus Mailing
In addition to specifying a mechanism for delivering messages to
user's mailboxes, RFC821 provided additional, optional, commands to
deliver messages directly to the user's terminal screen. These
commands (SEND, SAML, SOML) were rarely implemented, and changes in
workstation technology and the introduction of other protocols may
have rendered them obsolete even where they are implemented.
Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers
MAY implement them. If they are implemented by servers, the
implementation model specified in RFC821 MUST be used and the
command names MUST be published in the response to the EHLO command.
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFCEditor function is currently provided by the
Internet Society.