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400 defer

2011-09-27 17:12:13

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?


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