420 4.2.0 Transient Failure during recipients lookup AmbiguousRecipientTransientException

@Vince DiNenna, Thank you for the reply and clarification. My firm is the recipient in this case. I'll work with the sender to see if we can't get this resolved and I understand we (they) might need to get Microsoft involved.

While researching this, I'm seeing that this "could" be caused by someone on the sending side adding a user on the receiving side as a Microsoft Teams guest in an innocent attempt to have a meeting with them. Something went wrong somewhere in that process, and this is causing the duplicate...

What I'm wondering now is.... If my user (Sam on the receiving side) knows he had a Teams meeting with (Karen on the sender domain) on or about the day this started happening. Can we call up (Karen) and ask her to undo whatever she did to add Sam as a guest - will that possibly fix this.

If this is a possible solution, it seems like it would be faster than getting Microsoft involved.

Yesterday the issue for my client was finally fixed by removing the duplicate guest users which due to them having identical ID's made e-mail delivery and deleting them impossible.

After a lengthy e-mail exchange with Microsoft starting January 9th and a Teams chat with a Wicresoft support employee in Singapore the issue was finally fixed when their backend team deleted the duplicates.

It is still unclear how these duplicated guest users ended up in EXO's database. I do know my client tried to create a list of users to send Christmas cards tomorrow which didn't work as expected and the client deleted the list. At that moment the duplicated guest users appeared.

Many hours of our time, which I dare not invoice to my client, and no confirmation from Microsoft that the issue was due to a bug at their side. I did however get an e-mail from Wicresoft thanking me for my valuable time and understanding. Moreover I now know how to use Powershell on my Mac.

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 ?

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.

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