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I've noticed that when we take down servers (including domain controller, or exchange), mail being delivered to VALID users by external servers get a 5.1.1 error as noted below.

2015-10-21.log:Oct 21 14:34:57 smtp1 sendmail[25335]: t9LFJ0tf010462: to=<[email protected]>, delay=03:15:57, xdelay=00:00:00, mailer=esmtp940, pri=455814, relay=x.x.x. [13.39.64.42], dsn=5.1.1, stat=User unknown

Is this normal behaviour for Exchange? If not, is there a way to change exchange's behaviour so it doesn't report "user unknown" errors? (Otherwise upstream servers get confused and cause problems)

(I tried to disguise some info in the above)

UPDATE: I tried shutting down the DC and could not generate a 5.1.1, and tried sending mail when the Exch server in startup/shutdown - but still couldn't generate a 5.1.1. There must be some combination of the above which does this. Hoping someone has ideas.

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  • Is that the exchange server log for the smtp transaction?
    – Drifter104
    Oct 21, 2015 at 23:34
  • The subject is locked, so I can't answer. But I think I have a possible cause for your specific error code. A 5.1.x can occur if a relay server does not find the user that you're attempting to relay a message to. So if your DC is down, and someone uses an smtp relay to send an e-mail to an internal contact, you will get a 5.1.x.
    – Reaces
    Oct 23, 2015 at 7:48
  • Just break the bestpractice and install the dc role on the server & install exchange on it.. no more syncro problem as it would be only 1 VM
    – yagmoth555 ♦
    Oct 27, 2015 at 1:43

1 Answer 1

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  1. There's no good reason that I can think of that you would need to shut down any of your servers in order to back them up. There is a plethora of options and solutions for backing up live servers.

  2. Exchange relies on Active Directory. It needs information from Active Directory. If Exchange can't communicate with a Domain Controller then Exchange isn't going to function properly.

  3. Stop shutting down your servers to back them up and you won't have this problem.

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  • That doesn't really answer the question - do you know if Exchange generates a 5.1.1 DSN when the DC is down? Or when exchange is starting/stopping?
    – TSG
    Oct 21, 2015 at 21:22
  • Do you KNOW that it generates a 5.1.1 when the DC is down? (A 5.x.x is a permanent error which doesn't make sense if a DC is unavailable - I would expect a 4.x.x error)..or are you guessing? I voted down your answer because it addresses everything EXCEPT what was asked (I did not ask about backup - you are making assumptions about my ability to backup a server)
    – TSG
    Oct 21, 2015 at 21:26
  • Are users ever prompted for re-login? I've had an issue related to username, Outlook was expecting domain\username but I was providing [email protected] and Exchange did not like that.
    – nGX
    Oct 21, 2015 at 21:34
  • I'm hoping someone more informed can answer the actual question posted. An unavailable DC should generate a 4.x transient error - since Exchange does not know if the mailbox exists or not.
    – TSG
    Oct 21, 2015 at 21:42
  • @Telium This is perfectly reasonable and, as far as I can tell, correct. You are causing the problem yourself by shutting down servers unnecessarily.
    – Michael Hampton
    Oct 21, 2015 at 21:51

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