Exchange 2010 has a distribution group setup called postmaster and the Email addresses associated with it are postmaster@example.com, postmaster@example.net etc

Internal emails sent to postmaster work fine and go to all members of the group.

However from external via SMTP I get an 550 5.1.1 error saying user unknown.

What have I missed?

All my other Dist Groups work internally and externally, its just this one.


  • Is this address allowed in your spam filter? I have seen that (especially with Barracuda) stop external mailflow to an address. – David V Sep 18 '13 at 15:19

Don't usually see the Transport setting for the postmaster email account set to a distro, but ok.

You're likely getting this from external users because there is still a check requiring the sender to be authenticated (default for distro groups).

See the pic below...and change the distro group accordingly. Then external senders should be able to email the distro's email address.

enter image description here

  • I'm pretty sure this would give the sender a different NDR, the verbage being something along the lines of not having permission to send to this recipient. The OP states the NDR is stating "unknown user". – joeqwerty Sep 18 '13 at 15:25
  • @joeqwerty - no, MS was lazy at relaying back the right NDR when the box was checked: support.microsoft.com/kb/2773786 – TheCleaner Sep 18 '13 at 15:28
  • Although, it could also be if they are running an external mail gateway SaaS or appliance. If it does an LDAP lookup (like Ironmail/Ironport) but for a user instead of just a proxyAddress or SMTP address then it could bounce back the NDR before it ever gets to Exchange. – TheCleaner Sep 18 '13 at 15:31

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