When a username has the domain name appended, this option selects the format. Not all options are available on all OS platforms, as some characters are not allowed in usernames. Whether the domain name is the full username, i.e. "virtualmin.com", or just the first part, i.e. "virtualmin", is dictated by the Domain name style in usernames option on the Module Config page.

The username@domain option is not recommended unless you are migrating users from another system that already uses it. The Postfix mail server has problems with SMTP authentication by users with an @ in the username, and requires that an extra Unix user be created for each mailbox for mail delivery to work.