A customer was sending mail to a remote host and got this for a RCPT
TO response and wanted to know what it meant:
17:25:47.884 C: RCPT TO:<m(_dot_)xxxxx(_at_)xxxxxxxx(_dot_)com>
17:25:47.962 S: 400 defer
Because of the 4yz response, our MTA queued the message for a retry,
but I never seen this 400 and not sure what it suppose to mean. 5321
says:
x0z Syntax: These replies refer to syntax errors, syntactically
correct commands that do not fit any functional category, and
unimplemented or superfluous commands.
and it doesn't appear the address was incorrect.
I looked for the word "defer" and the only thing that "came close" was
in section 7.2
Whether disabling VRFY provides any real marginal security depends on
a series of other conditions. In many cases, RCPT commands can be
used to obtain the same information about address validity. On the
other hand, especially in situations where determination of address
validity for RCPT commands is deferred until after the DATA command
is received, RCPT may return no information at all, while VRFY is
expected to make a serious attempt to determine validity before
generating a response code (see discussion above).
What would make of this "400 defer" response?
TIA