When creating "extra administrators" for any particular domain, the domain's naming conventions for creating new users are not followed. For example, I use the format user.domain.com. However, if I create a extra admin in that domain, it creates the account with only the username portion, not the domain suffix.
Granted, these are only Webmin user accounts, not system accounts. However, this still presents a problem, since domain owners are creating their own extra admins with names like "bill" and "bob". Soon enough, there will be conflicts since those names will already be taken by other domain owners. Therefore, these usernames should respect the same naming conventions as any other user in the domain.