Resolving Exchange Assistants "TransientMailboxException" error, Event ID 9041

I recently encountered a mail flow problem in an Exchange 2016 environment, that was being caused by the DNS configuration on the Exchange server’s network interface.

The Application log of the Exchange server was flooded by event ID 9041:

Log Name: Application
Source: MSExchange Assistants
Date: 10/6/2016 4:56:46 PM
Event ID: 9041
Task Category: Assistants
Level: Warning
Keywords: Classic
User: N/A
Computer:
Description:

Service MSExchangeSubmission. An exception has been thrown: Microsoft.Exchange.Assistants.TransientMailboxException at Microsoft.Exchange.Assistants.Util.TraceAndThrow(Action function, AIException aiException, String nonLocalizedAssistantName)
  at Microsoft.Exchange.Assistants.Util.CatchMeIfYouCan(Boolean translateToPermanentException, Action function, String nonLocalizedAssistantName, ISet`1 permanentExceptions)
at Microsoft.Exchange.Assistants.Base.CatchMeIfYouCan(Boolean translateToPermanentException, Action function, String nonLocalizedAssistantName, ISet`1 permanentExceptions)

Looking into the network settings led me on to the root cause; the network adapters DNS settings was misconfigured and included an external DNS server:

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I removed the external DNS entry, so only the internal domain controller and DNS server was used.

After rebooting the server all mail flow errors was resolved, and the Application log cleared up again.

Remember, Exchange relies heavily on internal Active Directory and DNS services to support many internal services, including routing and mail flow.

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