Survey of SMTP implementations
RFC 876

Document Type RFC - Unknown (September 1983; No errata)
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Network Working Group                                       D. Smallberg
Request for Comments:  876                                           ISI
                                                          September 1983

                    Survey of SMTP Implementations


This memo is a survey of implementation status.  It does not specify an
official protocol, but rather notes the status of impementation of 
aspects of a protocol.  It is expected that the status of the hosts
reported on will change.  This information must be treated as a snapshot
of the state of these implementations.


     From May to August 1983, I tested SMTP servers on the Internet to
see whether they accepted connections from the Arpanet (a Class A
network) and ISI-Net (a Class B network), whether they accepted the user
"postmaster" as a mail recipient, and whether a nonexistent user was
immediately rejected as a mail recipient.

     The hosts from which the tests were conducted were ISI-VAXA on the
Arpanet (running 4.1bsd UNIX), and ISI-MOE on ISI-Net (running 4.1a).
Internet hosts were tested at various times throughout the last four
months.  During the survey, I noted anomalies in a few dozen hosts' SMTP
servers; examples included a RSET command causing the server to close
the connection, a VRFY POSTMASTER evoking a reply containing an illegal
mailbox, and some cases of improper reply codes.  These bugs were
reported and in most cases promptly fixed.

     I would class three problems as significant because about 40 hosts
exhibit at least one of them:

     1)  In reply to a RSET and/or a NOOP command, some servers reply
         "200", which is never a legal reply code, instead of "250".
         (See sections 4.2 and 4.3 of RFC 821.)

     2)  If a VRFY command occurs before a MAIL command, some hosts
         reply "554 Nested MAIL command".  The end of section 4.1.1 of
         RFC 821 states that a VRFY may occur anywhere in the session.

     3)  If a mail transaction is started, with a sender and receiver
         specified, and a RSET is issued before the text of the message
         itself is collected, some servers send a message to the sender
         about being unable to deliver mail because no message was
         collected.  While RFC 821 doesn't rule this out, it certainly
         is not consistent with the notion of resetting the transaction.

     In the table in the appendix, the names and addresses of the hosts
tested were taken from the NIC host table of 17 August 1983.  TACs and
echo hosts were not included in the survey.

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RFC 876

     Here are the summarized results of the survey:

483 hosts were tested

283 are claimed by the host table to support SMTP
 49 of those 283 (17%) failed to permit a connection to be opened from
      either ISI-VAXA or ISI-MOE.
 51 hosts did not claim to support SMTP, but did allow a connection to
      be opened from at least one of the two ISI test hosts.

285 hosts were connected to from ISI-VAXA
170 hosts were connected to from ISI-MOE; all 170 were connected to
      from ISI-VAXA as well.
115 hosts out of the 285 (40%), therefore, could be connected to from
      ISI-VAXA only.

 69 of the 285 connectable hosts (24%) returned a positive reply to the
      command "VRFY postmaster"

162 hosts out of the 285 connectable hosts (57%) immediately rejected
      mail addressed to a nonexistent user; that is, they gave an
      "unknown user" reply to the command "RCPT TO:<jqkxwzvb@host>",
      where "host" was the foreign host.
115 hosts out of the 285 (40%) gave a positive acknowledgement to a
      RCPT command with a nonexistent user.
  8 hosts (3%) were never up during this part of the test.

121 hosts out of the 162 which immediately reject mail to nonexistent
      users (75%) accepted mail for the recipient "postmaster".  Thus,
      42% (121 out of 285) of the connectable hosts do not immediately
      reject mail for "postmaster".


RFC 821  Postel, J., "Simple Mail Transfer Protocol", RFC 821, Network
      Information Center, SRI International, Menlo Park, August 1982.

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RFC 876

     The hosts in this table are taken from the NIC host table of
17 August 1983, with TACs and echo hosts omitted, and are grouped by
network.  There are six result entries for each host:

Claim SMTP    + = the host table lists this host as supporting SMTP;
              - = such support is not listed

Arpanet       + = a connection could be opened from ISI-VAXA
              - = no such connection could be opened

ISI-Net       + = a connection could be opened from ISI-MOE
              - = no such connection could be opened

VRFY Post     + = the command "VRFY postmaster" evoked a positive reply
              - = it did not

RCPT Post     + = the command "RCPT TO:<postmaster@host>" elicited a
                        positive reply
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