When logging into Usermin, e.g. to check their mail, users must enter their fully-qualified domain name as their username. (Note: We create all accounts with the format user.domain, but this would also apply to the user@domain format.) Entering this information is redundant when the URL used to access Usermin already contains this information. For example, if we create the DNS alias webmail.domain, users can go to the URL https://webmail.domain:20000/ to access their mail. Usermin should automatically detect the domain portion and accept just the user portion as their Username, appending the rest as necessary when actually logging into the mail server.
Note that SquirrelMail has worked this way for several years using the "Login Manager" (formerly "Vlogin") plugin. For more information, see:
http://www.SquirrelMail.org/wiki/VirtualHosts
and
http://www.SquirrelMail.org/plugin_view.php?id=47
This could also apply to Webmin/Virtualmin, but it is primarily a Usermin issue because of the large potential user base logging into it as a mail client.