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

New 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?

5 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.

Hello, @Jaycenornin my advice is to register the subdomain using the third-party GoDaddy service. After it, you can add it to your Office 365 instance and configure the DNS options.


Make sure you are not letting O365 manage your DNS configuration with NS records. Actually, you should not have any problems even with 900 subdomains.


Check a helpful link on your issue:


And another article on the official Microsoft website:


Another recommendation is to use Nakivo solution for Office 365 backup, it will help to recover your organization's data in cases of system glitches.