Some sanity checks:
1] Is the domain admin assigned to the proper domain?
You assign this with the drop-down when you create the admin, you can also change it by editing them in the list. Make sure the domain is checked.
2] Does the domain have quota and mailboxes available?
It’s possible to create email boxes that exceed the domain’s own quote, as system-admin, but the domain-admin cannot override it in this way, so this can cause issues.
