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
    2025-01-23T14:37:25+00:00

    @mikee p, Issue is most likely on your end not the recipient side. In my case an external party was invited to join as a Teams guest. Somewhere there's conflict with the email address Teams creates and the guest's external address (multiple email IDs). When emailing the guest at their external/business email, your email system sees two email addresses and doesn't know which address to use and fails as ambiguous email address. That's my understanding.

    After fighting with MS support, I finally copied their manager and the ticket was escalated to Exchange engineering, which is what Bertrand Muffuh suggested originally in this thread. The first level support requested the same reports and info over and over. Finally, they blamed our SPF records, which are fine. Enough already...

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  2. Anonymous
    2025-01-23T14:16:20+00:00

    We're trying to troubleshoot this as well. Appears to have started on Jan 8th, 2025. After reading all the posts here, I'm still confused as to who's side is having the issue. Is it the recipient's side or the senders side?

    The County our office is in can send e-mails to everyone at our firm except 1 user. The e-mails to the affected user never reach our office and do not show up in a message trace.

    The County sees these messages on their end. Again, we (the recipient) see nothing.

    Reason: [{LED=420 4.2.0 Transient Failure during recipients lookup AmbiguousRecipientTransientException, Exception of type 'Microsoft.Exchange.Transport.Categorizer.AmbiguousRecipientTransientException' was thrown.};{MSG=};{FQDN=};{IP=};{LRT=}]

    Reason: [{LED=550 4.4.7 QUEUE.Expired; message expired};{MSG=};{FQDN=};{IP=};{LRT=}]

    Who should be trying to resolve this? The Sender or The Recipient ?

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  3. Anonymous
    2025-01-16T16:36:52+00:00

    Just tried to familiarise myself with PowerShell, and on a Mac too... This is the day...

    Anyway the result when trying to delete one of these duplicated guest user is not a surprise:

    PS /Users/pooh> Remove-MailUser -Identity [e-mail address to remove] -Confirm:$false

    Remove-MailUser: ||The operation couldn't be performed because '[e-mail address to remove]' matches multiple entries.

    Ernst.

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  4. Anonymous
    2025-01-16T16:06:36+00:00

    Hi Ernst, Louis, Vince,

    thanks for your reply!

    It seems to me that those duplicates were created simultaneously at the same time. At least in my case it is.

    I've did my own test of deleting 2 duplicates.

    When going after one unique ID, I receive this error:

    > Get-MailUser Guest_396blablablablaa111 | Remove-MailUser -Confirm:$False

    An Azure Active Directory call was made to keep object in sync between Azure Active Directory and Exchange Online. However, it failed. Detailed error message: Resource '396bla-bla-bla-blaa111' does not exist or one of its queried reference-property objects are not present. DualWrite (Graph) RequestId: d8dbla-bla-bla-blacbbd

    However, the correspondig AzureAD object did get deleted, but the two MailUsers are still there in EXO.

    So, I assume the "delete-request" from AAD back to EXO fails.

    Even purging the object from "deleted users" in Entra did not trigger the removal of the MailUsers in EXO.

    I ended up in creating a case with our support contractor, who hopefully can escalate this to MS.

    @Louis and @Vince, maybe the above information is helpful for your cases too...?

    Or maybe a nice MS moderator here could also forward all the information collected to MS OPS teams!? :-) Please? :-)

    Cheers, Alex

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  5. Anonymous
    2025-01-16T15:18:33+00:00

    Thanks Louis, this helps us understand better what we are dealing with.

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