Hmm, it seems a little tricky and possibly not waterproof but ok,

If your server is also the mailserver, and if the login/usernames are like user@myserver.com, try the method that the great Dave Roth uses to retrieve user info, find out the user, and just concat the user/loginname @myserver.com.

The NetUser.pl Script should help you with how to do this.
And you find that one here: http://www.roth.net/perl/scripts/scripts.asp?NetUser.pl

Good luck,
Teabag
Sure there's more than one way, but one just needs one anyway - Teabag