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

Locked Question. This question was migrated from the Microsoft Support Community. You can vote on whether it's helpful, but you can't add comments or replies or follow the question.

0 comments

60 answers

Sort by: Most helpful
Most helpful Newest Oldest
  1. Anonymous
    2025-01-14T10:27:23+00:00

    In my case, it seems to be related to the creation of a new Team, as the Teams group was created about 40 minutes before.

    @vince, is this maybe the case for you?

    Cheers, Alex

    Was this answer helpful?

    0 comments
  2. Anonymous
    2025-01-14T09:15:40+00:00

    Hi Bertrand,

    Looks like in the Exchange online directory Guest users get added twice, though there's only one AzureAD object.

    I've got the same issue, here you can see one example:

    Please let your L2 / Escalation Engineer / Product Group know.

    Thanks,

    Alex

    Was this answer helpful?

    0 comments
  3. Anonymous
    2025-01-08T17:57:24+00:00

    I have sent multiple NDR, extended trace reports and actual failed emails to MS for analysis. They are discussing the issue internally was the last update I received.

    Was this answer helpful?

    0 comments
  4. Anonymous
    2025-01-08T17:34:58+00:00

    Are we all still waiting on the same issue getting fixed? This is driving us mad. The only work-around we were able to do is make alias's on the receiving user's domain and we are sending to that alternate address since only the SMTP path is broken to the exact user (two of them). Such a frustrating situation.

    Was this answer helpful?

    0 comments
  5. Anonymous
    2024-12-21T20:14:34+00:00

    I have the same exact issue. Identical symptoms. Have not found a solution yet, working with a technician. Ours started Wed on the 18th? (then wasn't happening for a few hours yesterday afternoon) back again a few hours later.

    One thing I found: If you went to list guest users in the main admin panel, the users with the issue were the only people who's "Display Name" was set as their email address.

    Changed that to a first name, but this did not fix the issue. Also deleted guest user from Azure Active Directory Guests, also did not fix

    Was this answer helpful?

    0 comments