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
    2025-02-06T20:16:34+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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  2. Anonymous
    2025-02-06T15:39:26+00:00

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

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  3. Anonymous
    2025-02-06T12:18:41+00:00

    Alex,

    We have this exact issue. Do you (or anyone else that had the issue fixed) have a case number we could provide to support? I've pointed to this thread and it may be helpful to have that information as well. Thank you!

    Gene

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  4. Anonymous
    2025-02-05T20:28:14+00:00

    They keep asking to fix DKIM record. That doesn't come into play until after an email leaves our email server path. Messages do not arrive at our email security server. They don't make it out to 3 recipients. We send to 1000's of recipients daily. Only three don't receive our emails. They only respond to my management but then ask us to do the same things over and over. Makes no sense. They are dragging their feet. Is there a lack of peoplepower to fix all the issues? Only thing I can think of. Perhaps need a higher-level paid SLA?

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  5. Anonymous
    2025-02-05T20:11:23+00:00

    My issue is still escalated... they keep asking me to "try it now" and my messages still sit as deferred and i still see the duplicates. what is the trick to getting them to fix this?

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