An example client-server conversation is given at the end
of this document.
In SMTP server EHLO replies, the keyword associated with this
extension is XCLIENT. It is followed by the names of the attributes
that the XCLIENT implementation supports.
The XCLIENT command may be sent at any time, except in the
middle of a mail delivery transaction (i.e. between MAIL and DOT,
or MAIL and RSET). The XCLIENT command may be pipelined when the
server supports ESMTP command pipelining. To avoid triggering
spamware detectors, the command should be sent at the end of a
The syntax of XCLIENT requests is described below. Upper case
and quoted strings specify terminals, lowercase strings specify
meta terminals, and SP is whitespace. Although command and attribute
names are shown in upper case, they are in fact case insensitive.
xclient-command = XCLIENT 1*( SP attribute-name"="attribute-value )
attribute-name = ( NAME | ADDR | PROTO | HELO )
attribute-value = xtext
Attribute values are xtext encoded as per
The NAME attribute specifies an SMTP client hostname
(not an SMTP client address), [UNAVAILABLE] when client hostname
lookup failed due to a permanent error, or [TEMPUNAVAIL] when
the lookup error condition was transient.
The ADDR attribute specifies an SMTP client numerical
IPv4 network address, an IPv6 address prefixed with IPV6:, or
[UNAVAILABLE] when the address information is unavailable.
Address information is not enclosed with .
The PROTO attribute specifies either SMTP or ESMTP.
The HELO attribute specifies an SMTP HELO parameter
value, or the value [UNAVAILABLE] when the information is
Note 1: syntactically valid NAME and HELO attribute-value
elements can be up to 255 characters long. The client must not send
XCLIENT commands that exceed the 512 character limit for SMTP
commands. To avoid exceeding the limit the client should send the
information in multiple XCLIENT commands; for example, send NAME
and ADDR first, then HELO and PROTO.
Note 2: [UNAVAILABLE], [TEMPUNAVAIL] and IPV6: may be specified
in upper case, lower case or mixed case.
Note 3: Postfix implementations prior to version 2.3 do not
xtext encode attribute values. Servers that wish to interoperate
with these older implementations should be prepared to receive