When sending out emails with a MAIL FROM address > 81 characters I receive bounce messages from some MTAs:

501 Syntax error - Badly formatted address. (in reply to MAIL FROM command)

As soon as I change the MAIL FROM address to <= 81 characters, the MTA accepts my message.
I didn't find any limits in the RFC docs.

Is this expected behaviour or is the recipients' MTA misconfigurated?


Accoding to the answers given at


The answer is 254 according to RFC 5321

So, those MTA are badly configured.

  • That's the length of a valid email address. Maybe it have something to do with the SMTP protocol or a different restriction though? – horen Jun 4 '14 at 14:29
  • 1
    No, it has not. A SMTP server is magically required to actually acccept valid email addresses, or? How else would it work? – TomTom Jun 4 '14 at 14:30

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