For the built-in default user type User (default) their exists the base attribute Username.
This attribute carries the format restriction of Email format.
Since the following question was ask and closed sometime ago, was hoping to revisit this.
Can a sign-on be force to require the the fully qualified Username
<username string>@example.com vs <username string>.
It calls into question the assigned behavior when more than one user exists with that username but different email domains.