Domain name style in usernames
This option determines whether the domain name used in usernames that contain the domain will be the full domain name, i.e. "virtualmin.com", just the first part, i.e. "virtualmin", or the user/group name for the domain. By default, the group name is used for new top-level servers, and the first part of the domain is used for sub-servers.

In some environments, it may be easier for users to understand a username like "joeuser@virtualmin.com" for their POP/IMAP or FTP login name than "joeuser.virtualmin".