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
    2025-02-14T21:36:35+00:00

    I had the same issue. They fixed my issue in a couple days (I did this through the 365 partner portal however) - did you make a ticket within the admin portal for this?

    I linked them this post itself & they thanked me for it!

    They told me they've contacted the 365 developer team about this as it has been happening more and more often. Who knows how long a full fix will take though

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  2. Anonymous
    2025-02-14T17:49:15+00:00

    Thanks for sharing the bug number. I've been with MS for 3 weeks, and no support
    This is very frustrating

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  3. Anonymous
    2025-02-08T17:51:54+00:00

    Thanks for the bug#, Matt!

    Much appreciated!

    Cheers, Alex

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  4. Anonymous
    2025-02-06T21:57:21+00:00

    Microsoft told me to ask it to be elevated to the PG group if it happens again. This was through MS support starting with my E365 partner initially.

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  5. Anonymous
    2025-02-06T21:11:24+00:00

    2 days? I'm on week 9. What channel or method did you use. I'm fighting nearly every day for better support and escalation. Driving us nuts.

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