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I am in the middle of a Cutover Migration from Exchange 2010 to Exchange online.

Accidentally for about half a day, i have changed the priority for the Exchange Online MX record to be the lowest instead of the on premises Exchange so their e-mail couldn't work as expected.

Thing is while i've changed the MX priorities to 0,200 and 300 instead of 100,200,300 which was the previous default priorities with the on premises MX record to be the lowest one, i am still getting some of the emails back sometimes when sending to this domain (i can always send from Gmail though without a problem). The email bounces back and if i resend it then it finally reaches its destination.

Please note that using NSLOOKUP and have set type to MX i can see that the DNS records have been propagated to the correct ones however i still getting those errors.

My question is how this can still happen while i have changed the DNS records about 2 weeks ago? Is there something i am missing here?

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  • Check from an external source, like mxtoolbox.com
    – DanBig
    Feb 10, 2015 at 14:06
  • @DanBig I have already done this, it shows me the correct priority propagation. It is like the result i am getting when using NSLOOKUP using Google DNS. Thanks for the suggestion though.
    – Konstantinos Xanthopoulos
    Feb 10, 2015 at 14:09
  • I don't understand the problem. What's happening exactly? Users aren't receiving email at the on premises Exchange server? What do you mean you're getting some of the emails back? Do you mean you're getting an NDR for emails your sending from an external address to an Exchange user? If so, what is the NDR?
    – joeqwerty
    Feb 10, 2015 at 15:24
  • @joeqwerty Users are able to receive messages most of the times but for some reason some times mail returns back with a 550 #5.1.0 error because it tries to locate the mailbox on Exchange Online instead of on premises Exchange 2010. This makes sense as none mailbox have been migrated to the cloud. But my question is, why it doesn't always lookup first on the lowest priority MX record and looks on Exchange Online MX record hence it fails? If i send it two or three times the message arrives but i can't explain that behaviour.
    – Konstantinos Xanthopoulos
    Feb 10, 2015 at 21:38
  • Could it be that the sending servers can't connect to the primary MX and are delivering to the secondary MX as a result? If so, removing the secondary MX records will force the sending servers to queue the email for later retries to the primary MX.
    – joeqwerty
    Feb 10, 2015 at 22:01

1 Answer 1

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Could it be that the sending servers can't connect to the primary MX and are delivering to the secondary MX as a result? If so, removing the secondary MX records will force the sending servers to queue the email for later retries to the primary MX

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