RFC 1869 (rfc1869) - Page 3 of 11

SMTP Service Extensions

RFC 1869                SMTP Service Extensions            November 1995


4.  The EHLO command

   A client SMTP supporting SMTP service extensions should start an SMTP
   session by issuing the EHLO command instead of the HELO command. If
   the SMTP server supports the SMTP service extensions it will give a
   successful response (see section 4.3), a failure response (see 4.4),
   or an error response (4.5). If the SMTP server does not support any
   SMTP service extensions it will generate an error response (see
   section 4.5).

4.1.  Changes to STD 10, RFC 821

   This specification is intended to extend STD 10, RFC 821 without
   impacting existing services in any way.  The minor changes needed are
   enumerated below.

4.1.1.  First command

   RFC 821 states that the first command in an SMTP session must be the
   HELO command. This requirement is hereby amended to allow a session
   to start with either EHLO or HELO.

4.1.2.  Maximum command line length

   This specification extends the SMTP MAIL FROM and RCPT TO to allow
   additional parameters and parameter values.  It is possible that the
   MAIL FROM and RCPT TO lines that result will exceed the 512 character
   limit on command line length imposed by RFC 821.  This limit is
   hereby amended to only apply to command lines without any parameters.
   Each specification that defines new MAIL FROM or RCPT TO parameters
   must also specify maximum parameter value lengths for each parameter
   so that implementors of some set of extensions know how much buffer
   space must be allocated. The maximum command length that must be
   supported by an SMTP implementation with extensions is 512 plus the
   sum of all the maximum parameter lengths for all the extensions
   supported.

4.2.  Command syntax

   The syntax for this command, using the ABNF notation of [2], is:

     ehlo-cmd ::= "EHLO" SP domain CR LF

   If successful, the server SMTP responds with code 250. On failure,
   the server SMTP responds with code 550. On error, the server SMTP
   responds with one of codes 500, 501, 502, 504, or 421.





Klensin, et al              Standards Track