Wonder if any workaround can create actual admin@ mailbox?
I have domain, for example @abc.com created. And now would like to add domain @def.com. When I tried to add admin@def.com it said
“You may not make a user account for that address because it is frequently used for domain control validation. Use an alias instead if necessary.”
I think you normally have one admin@domain.com (the initial domain when you installed MIAB) and use that for admin purposes on all your domains. Then for each new domain, you just start by creating a real user on that domain - all admin work being done by the one initial admin@domain.com user.
I guess you could create a dummy user to trigger new domain creation, but that would not be an admin user and would not be called admin. (You can grant admin privileges to other users, but that’s not quite the same as being admin@domain.com.)