I'm currently running

on Windows Server 2019

I have used 85 out of our licensed maximum of 250 mailboxes.

In the Accounts tab for a domain, when I try to create a new user or alias, I get a "Name <username> already in use." message and I am prevented from creating the new user. There are no current aliases set up under that domain and that particular domain only has 17 users out of a maximum of 100 users for the domain. All of the existing users for that domain can fit on one page of the listing of users for the domain, so I am sure that I am not duplicating an existing username.

This may be the first time I've tried to create a new domain user since I migrated to Windows Server 2019 on a new server. I have also tried creating a new user or alias under other mail domains that we host. I get the same result regardless of which domain I try to create the new user in.