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
    2025-01-14T09:51:22+00:00

    well, I got 10 duplicates...

    Found them by running:

    $a = Get-Recipient -RecipientTypeDetails GuestMailUser -ResultSize unlimited # might take loooooong... ;)

    $a | Group-Object ExternalEmailAddress | sort count -desc | select -first 20 | ft -au count,name,group

    If the property "Group" contains more than one GUID, you've got more than one recipient in your system with the same ExternalEMailAddress.

    Cross-check:

    $a | Group-Object primarySMTPAddress | sort count -desc | select -first 20 | ft -au count,name,group

    Let's dig a bit...

    Just get all into one object when there is a count greater than 1:

    $b = $a | Group-Object ExternalEmailAddress | ? count -gt 1

    Let's check, when whose have been created or changed:

    $b.group | ft -au name,externalemailad*,whenc*

    So my duplicates had all been created on 16.12.2024 17:25 UTC (or the other way: 2024-12-16 5:25pm UTC).

    Anyone else at this time or close to?

    Best, Alex

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  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

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  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.

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  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.

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  5. Anonymous
    2024-12-23T19:46:12+00:00

    We also started experiencing the same issues last week. MS Support is still working on the issue. I thought it might be guest users as well, so I deleted the accounts in question, but it has not resolved the issue.

    I believe that the reason deleting the users didn’t fix the issue is that the guests still remain in the “contacts” section of the exchange admin panel. Also in my case the two broken are also duplicated.

    I can’t delete them any further on my own using power shell or guidance from MS yet but it is easier to test for the issue by running message traces as it shows the “pending” status within 1 minutes of sending if you extend the reporting time past the current time when you run the report.

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