I recently encountered a mail flow problem between two of my test lab environments that was being caused by the DNS configuration on the receiving server’s network interface.

While I was trying to send email from organization A to organization B, the emails were queuing on the organization A transport servers with the following error:

451 4.7.0 Temporary server error. Please try again later. PRX2

Here’s the full Exchange management shell output:

[PS] C:\>get-queue ex2016srv211 | Get-Message | fl

RunspaceId                      : 524470e8-3e28-4260-8f5b-64c4749244af
Subject                         : Test 2
InternetMessageId               : <564407fa60df4720851f66fc31dabb8a@EX2013SRV1.exchangeserverpro.net>
FromAddress                     : Alan.Reid@exchangeserverpro.net
Status                          : Retry
Size                            : 6.833 KB (6,997 bytes)
MessageSourceName               : SMTP:Default EX2016SRV2
SourceIP                        :
SCL                             : -1
DateReceived                    : 6/01/2016 2:25:41 PM
ExpirationTime                  : 8/01/2016 2:25:41 PM
LastError                       : 451 4.7.0 Temporary server error. Please try again later. PRX2
RetryCount                      : 1
Recipients                      : {administrator@exchange2016demo.com;3;2;[{LRT=};{LED=451 4.7.0 Temporary server
                                  error. Please try again later. PRX2 };{FQDN=};{IP=}];0;CN=To Partner Company
                                  2,CN=Connections,CN=Exchange Routing Group (DWBGZMFD01QNBJR),CN=Routing
                                  Groups,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative
                                  Groups,CN=Exchange Server Pro,CN=Microsoft

What I found interesting was that internal mail flow for organization B was not working either. On closer inspection of that Exchange server I found the following Application event log entry repeating every 5 seconds or so.

Log Name:      Application
Source:        MSExchange Assistants
Date:          1/6/2016 4:05:17 PM
Event ID:      9041
Task Category: Assistants
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      DEMOEX16.exchange2016demo.com
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)

The server happened to be running the Preview release of Exchange 2016, so I took the time to perform an upgrade to Exchange 2016 RTM before anything else. When the problem persisted, I dug further and found that I had configured the network interface on the server with a secondary DNS of the IP address of my home router.


After removing the extra DNS server so that only my Active Directory DC/DNS servers were being used, and then rebooting the server, mail flow between the two organizations and internally within organization B began working without error. Exchange transport is sensitive to DNS issues, and this seems to be just another case of that.

About the Author

Paul Cunningham

Paul is a former Microsoft MVP for Office Apps and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul no longer writes for Practical365.com.


  1. Jeremy

    Just ran into this! Running 2016 CU8 and coexistence with Exchange 2010. Any mail flow to/from the 2016 server wasn’t working. Interesting how the primary DNS server (internal DC) being up and available doesn’t help matters. Just having the secondary DNS populated, in this case with a public DNS server’s IP, caused the issue.

    It would be interesting to know if it is that both primary and secondary DNS need to be AD-integrated (or both not), or what.

    Thanks for having the easy fix published!

  2. Tony

    Ran into this on a new lab system. You saved me some head scratching. Thanks for sharing.

  3. Mike

    Nice one! I had this issue. Weird how secondary DNS would prevent emails being submitted.

  4. Hesan

    Hello Paul,
    I also checked my DNS settings and everything is fine. Still the MSExchange Transport service wont start and i am continously getting these warnings of 9041.

  5. sajid

    i dismount the db and mount it again, and it fix the issue.

  6. Ronald Graham

    I am having the same issues and errors but my DNS is good. I have been fighting with this for a month now. the issue comes and goes with no changes to DNS. A restart usually fixes the issue but I can’t always restart he server everything I have this issue popup. I think it may be a bug in 2016.

  7. Ramy Othman

    Dear paul,,

    i have same Error with my Server Exchange 2016 also ,, and dont use Router DNS ,,
    i make , Restart for all Exchange Services , and restart Server,, Restart DC and ADD server ,, and nothing happen -_-

    i dont know why this problem happen ??

    i also try to Delete all VM ,, and build lap again ,, and after i changed Virtual Directory with Certification from Internal CA ,,

    that Error happen again -_0


  8. Phil Goldwasser

    Hi Paul, this is almost exactly the problem I had written to you about. you responded to check DNS and the event logs, so I went into the event viewer and found this error. The only difference is that I get PRX4 at the end of the “temporary server error” message and not PRX2. I double checked DNS and it is set correctly. I have one DNS server and all three Exchange 2016 servers have that DNS set for the single adapter that I am using on each server. Any other adapter on the servers are disabled. Right now two of my servers are throwing this error, while one is not. I am about to start going live, and I cannot do that until this is cleared up.

  9. Helao

    Paul…cant you create an app for this website…its a massive help

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