Section Appendix F. Deprecated Features of RFC 821

A few features of RFC 821 have proven to be problematic and SHOULD
   NOT be used in Internet mail.
F.1.  TURN
   This command, described in RFC 821, 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
   RFC 821 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 RFC 1123, 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 RFC 1123.  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 SHOULD be used rather
   than 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
   RFC 821 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 header 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, RFC 821 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 RFC 821 MUST be used and the
   command names MUST be published in the response to the EHLO command.
Author's Address
   John C. Klensin
   1770 Massachusetts Ave, Suite 322
   Cambridge, MA  02140
   USA
   EMail: john+smtp@jck.com
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