Exchange 2013 SMTP receive connector error 554 5.4.4

Last Updated: 8/1/17

Symptoms

emails are not received or are received with a delay. sending mail server received error: 451 4.7.0 Temporary server error. Please try again later. PRX2 (Reason:451 4.7.0 Temporary server error. Please try again later. PRX2 )

Error message from Exchange 2013 logs: Message or connection acked with status Fail and response 554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain.
(Log file location: c:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpReceive)

Cause

This started happening on the same day the we implemented split DNS (seperate external and internal DNS records). Since we were significantly reducing which internal records were made available externally; I believe that this is what caused the problem. Normally this would not cause an issue except that we had both external (read: public) and internal DNS servers on the exchange server NIC.

Resolution

remove external DNS servers from exchange server NIC and flush your DNS cache.
Note: we are still in the middle of this, so this solution is not fully confirmed.

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