Shared mailbox in Secondary Domain creates user with Primary Domain

Haneef Ibn Ahmad 

I just ran across this today.... FIVE YEARS AFTER YOUR POST. lol

It's absolutely bizarre. 

So if you setup a shared mailbox using sharedgroup @ any domain attached to the account, an alias is automatically setup for sharedgroup @ primary domain.

Once this is done, you can no longer setup anything else with sharedgroup @ on ANY domain no matter how many you have as they all create this mystery alias in the background and fail because it's already created elsewhere.

If you add an additional alias it will allow you to delete the @primarydomain as an alias....you think you've solved it. But when you visit again you'll see that it's returned.

The fix mentioned above no longer works. Apparently MS "fixed" that and it no longer works. It reappears after you save just as if you didn't do anything at all. 

Anyways, although the error says so, the problem is not because the domain being setup is in use elsewhere, it's because your boxname@ is in use by your primary domain. 

If you have multiple domains you can normally duplicate users for each domain. But not on a shared mailbox. Once you do, regardless of the domain it's setup on, that username can no longer be used across all of the domains in the account.