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
    2025-01-30T05:06:46+00:00

    My client is experiencing the same issue. We've been waiting two weeks for MS Support to resolve. There has been no response to my many requests for a case update other than "We might need to run the back-end diagnostics for your tenant account," and no follow up. My experience with MS Support has been pretty good in the past but they really seem to be dropping the ball on this one.

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  2. Anonymous
    2025-01-30T08:40:09+00:00

    Good morning everyone,

    According to MS, the root cause was a synchronization issue between Azure and Exchange directories.

    My assumption was confirmed.

    Cheers, Alex

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  3. Anonymous
    2025-01-30T10:37:34+00:00

    @AlexC2232 & @Vince DiNenna & Everyone else....

    Update:

    Just to update the thread... I explained to the County's IT department that I believed the issue was on their end. I pointed them to this thread, and they reached out to Microsoft. It only took a day and Microsoft resolved the issue. I do not have official word yet as to the root cause and I may never get that; however, I suspect it's the syncing issue @Alex indicated in his earlier post.

    The main point of my post is - this was done rather quickly in my case. Was it because Microsoft is now aware of this issue and can address it more quickly? Does the County I'm working with have some sort of higher tear support? Did others get the B Team at Microsoft, and we got the A Team?

    The world may never know.

    Super special THANK YOU to @Vince & @Alex! Great work! We thank you. And I guess thanks to @Microsoft too.

    Good Luck to those of you still dealing with this - or have reached this page with the same issue.

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  4. Anonymous
    2025-01-30T10:39:30+00:00

    Thanks for all the updates Alex! Cheers!

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  5. Anonymous
    2025-01-30T11:15:17+00:00

    Hi @Mikee!

    Glad to hear they eventually got it fixed. Happy to help!

    From my experience, you've got two support tiers at MS: professional support and premier support.

    The latter you have access to with an Enterprise Agreement and/or inserting extra coins...

    Premium support can be really fast, as they do have other SLAs than professional support. (my experience and what I've been told) ... I guess, you may call them "A Team". ;o)

    Professional support works, like, "I'll do my very best, Ms. Sophie", but it seems to me, there is no pressure on them to get things sorted quickly... I might be wrong, though that's my impression. Moderators / MS support shall correct me, if I'm wrong. :)

    Like you said, quote: Good Luck to those of you still dealing with this - or have reached this page with the same issue.

    Cheers everyone, Alex

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