When you create a new domain, the display name of the domain admin will be the username you entered.
If you go manage the domain, and add users without giving a fullname, the display name will be user@domain.com
Maybe the domain admin should also be saved in admin@domain.com format to make it consequent.
Also if the domain admin starts mailing without changing a setting, a display name of admin@domain.com would be better than just "admin" for the one who receives the mail.
Not really a problem, but just a consistency thing.