RFC 1869 (rfc1869) - Page 3 of 11
SMTP Service Extensions
Alternative Format: Original Text Document
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