New subdomain but e-mail addresses are already in use?

Copper Contributor

I just created a new subdomain in our M365 for special purpose e-mails & 3rd party mailing support. When I attempt to create a new shared mailbox address@subdomain.domain.tld I get the error that the e-mail address is already in. The group in question, address@domain.tld, is an AD synced distribution group and most certainly is NOT using the subdomain in any aliases. I've checked EOL powershell get-recipient and there is no recipient for address@subdomain and it's not associated with any SharePoint sites or Teams.


In M365 Admin Center, when I attempt to create the shared mailbox, I get the error that it is in use by the group "address", which the admin center calls a "null team", but is the AD synced DG by its primary e-mail address.


In EOL EAC if I attempt to create the shared mailbox there, it just says the "The email address is already being used."


This is a brand new, never been heard of, never been seen before subdomain. Created in M365 as a new domain (cause it's easier than trying to add subdomain support in EAC and easier to manage). What gives?

4 Replies
Is this an Exchange hybrid scenario?
Not hybrid. AAD Connect synced and pw synced (one-way) but not EO hybrid.
Wouldn't make a difference, tho, since the address@subdomain recipient does not exist anywhere on-prem.
Hi @Jaycenornin,

What happens if you send an email to this address? Do you get a bounceback notification? If not, can you do a message trace against the test message and see what became of it?
Of course there's an NDR.
I've already deleted and recreated the DG on-prem in order to free up the address on the subdomain. The work I needed to get done is done.

The problem is that this is an issue with something in M365 reserving addresses for subdomains. It's not an error or a hidden recipient, it's a thing with how M365 works. I want to know what and why.