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
    2025-02-07T17:56:51+00:00

    It look 9 days from opening a ticket with our license vendor to resolution. This is from the last email I got.

    "Issue Definition: Unable to send email to guests

    Resolution: 
    The reported issues are being resolved by manually deleting the contacts from the backend.

    I also want to inform you that our product group is working on a permanent fix. A bug has been reported and is currently being addressed."

    I have asked for some sort of reference number to the bug report that can be cited so the problem can be researched and addressed more quickly. I will post back if i get a useful reply.

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  2. 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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  3. 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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  4. Anonymous
    2025-02-06T20:16:34+00:00

    I had two addresses that this applied to. MS fixed one within two days but didn't the other one. So it took another two days when I kept on them. This thread pointed me down the right path so thank you! When I asked MS what could be done to prevent this they told me nothing - that it is a bug on their end. Hopefully it will get fixed soon.

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  5. Anonymous
    2025-02-06T15:39:26+00:00

    Thank you! Finally got confirmation that this was resolved this morning.

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