420 4.2.0 Transient Failure during recipients lookup AmbiguousRecipientTransientException

Hi Mikee,

yes, indeed, to my best knowledge and from what I've seen in my client's system, the issue started at the creation of an M365 group and/or adding external recipients to that group.

The situation I've reconstructed in my old cerebral processing unit:

• a user (sender side) created a new M365 group
• one or more external recipients got added to that group
• that triggered the creation of a Guest user object in Azure AD
• as this one should be a recipient in a distribution group (which is a function of the M365 group), a sync is triggered from AAD to EXO directory, where a MailUser / GuestMailUser is created.

--> and here seems to be the point where the error happened. At this stage for one external e-mail address two MailUsers were created.

(Note: I did not reproduce my theory to avoid anything else to break in this system.)

On the other hand, removing the duplicate MailUsers doesn't work, because (again, to my best knowledge and from what I've tested in my client's system):

• Remove-MailUser <external e-mail address> fails, because there are 2 objects returned. Needs to be a unique identifier.
• Remove-MailUser <distinguishedname of 1st MailUser> obviously calls upon AAD to delete the AAD object, which in turn creates another call to ExO directory to remove the MailUser. The latter somehow fails, which results in the MailUsers are still present, though the corresponding AAD object was deleted.

-> It can be found in the "Deleted Users" in EntraID.

-> Unfortunately no other sync seem to happen.

-> Even purging the object from Deleted Users did not trigger a sync. The MailUsers just stay.

• Trying to remove a MailUser in another attempt immediately fails with an error, saying, there is no corresponding object in AAD.

At this point I had to engage the support contractor.

I hope I have been able to explain my experience clearly.

Cheers, Alex

@Alex. Thank you for clarifying this to me and taking the time to reply. I really appreciate it. I wrote the reply below to Vince and wonder if you have any input on this additional question?

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.

@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.

Hi Mikee,

as Vince said, the issue is on the sender's system, as there was more than one MailUser created for the external e-mail address.

Like described previously, the sender's admin cannot remove those duplicates.

Unfortunately it looks like MS has to perform this task of removing the duplicate MailUsers.

I suggest raising a case with MS support.

Mine is in progress...

I hope they can fix the issue based on the information provided.

Cheers, Alex