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
    2025-01-30T18:58:59+00:00

    Thanks everyone for your work and documentation on this issue. I'm experiencing the same thing and was tearing my hair out until I found this.
    My ticket is escalated with M$ for them to cleanup this mess.

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  2. 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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  3. 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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  4. Anonymous
    2025-02-05T21:43:03+00:00

    Hi Matt,

    Try adding the Team Manager and the Technical Advisor from the signature of the case mail into the TO: address field.

    That could speed things up a bit, at least that's what I experienced in earlier cases...

    And point them to this thread again. It speaks volumes and shows that other guys at MS (or 1stL subs) already solved the issue for other customers.

    Crossing fingers for you!

    Cheers, Alex

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  5. 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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