420 4.2.0 Transient Failure during recipients lookup AmbiguousRecipientTransientException

Anonymous
2024-12-17T21:59:18+00:00

Exchange Online

Sending email to certain external email recipients expires. The email recipients are also a Guest Mail User invited to use/access SharePoint or Teams. Could the external email address and guest email be conflicting and causing our sending server to not resolve the external address? If yes, how can this be fixed without disrupting their SharePoint and Teams collaboration?

Had a ticket open with MS support. Had me run an extended trace report. MS support read the report and said the recipient server was not able to resolve the recipient email address. However, the IT team of the recipient server said it's our server that cannot determine the correct email address.

Ran another extended report but still running after many hours.

Reason: [{LED=420 4.2.0 Transient Failure during recipients lookup AmbiguousRecipientTransientException, Exception of type 'Microsoft.Exchange.Transport.Categorizer.AmbiguousRecipientTransientException' was thrown.} Reason: [{LED=550 4.4.7 QUEUE.Expired; message expired}

Any ideas?

Thanks,

Vince

Microsoft 365 and Office | Install, redeem, activate | For business | Other

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  1. Anonymous
    2025-03-06T13:21:38+00:00

    I had to make about 5 tickets before this was resolved. I believe the ticket/team that was finally able to help me was on the Azure side. I went into the Entra admin center and had to choose a non-correct option to get it to let me make a ticket. i then titled it "Out of sync AD and Exchange online broken Guest Email Contact"

    It still even when I did that took many weeks. Hope this helps

    And for anyone still reading this and being given the run around, Powershell can not fix this using the permissions of an admin of the tenant. It had to be fixed by an engineer on Microsoft's side. Your support rep that works with you is not going to be able to fix it, and they are likely going to do a TERRIBLE job of helping and following up, they will refuse to let you talk to their supervisor, they will fail to escalate. They will tell you they have escalated (they will have honestly done that) but it will seemingly go nowhere for weeks.

    My temporary solution in case I did not mention it here yet - was to have an admin (in my case also me because we own the other party even though we have separate tech stacks at the moment) make an alias on the other user's mailbox. And then encourage all users of my domain to email THAT alias. It was a pain because users can't hit reply to emails. It was also a pain because the ability to redirect mail using a rule is also broken because the filtering for a specific "recipient" also fails due to the duplicate.

    I hope all of this helps someone. I also started copying the MS Support rep's manager on all emails back.

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  2. Anonymous
    2025-03-06T05:56:00+00:00

    Its now cloesed, but it didnt fix anything and dont let me remove duplicate contacts (guests). So what now?

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  3. Anonymous
    2025-02-25T13:09:38+00:00

    Nice. We had our issue resolved last week after over two months of pushing. Thanks to everyone here!

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  4. Anonymous
    2025-02-19T13:39:26+00:00

    Yes, I opened it through the Administrator portal, I sent these links together. I had already made all the queries via Powershell, the responsible engineer requested the PSR twice, with the same steps. The feeling is that he has a lot of tickets and cannot respond, and he is buying time.

    They have already changed engineers twice

    They finally deleted the duplicate users. I don't know if it will happen again, but they blocked it so that only administrators could create groups in Teams, and guest users could use it. My problem was solved in 25 days. Apparently there will be an update scheduled to fix it in March of this year.

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  5. Anonymous
    2025-02-17T11:50:56+00:00

    Yes, I opened it through the Administrator portal, I sent these links together. I had already made all the queries via Powershell, the responsible engineer requested the PSR twice, with the same steps. The feeling is that he has a lot of tickets and cannot respond, and he is buying time.

    They have already changed engineers twice

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