Ok, so here's the deal. all the rest is sorted.
I have a server hosting multiple domains, and it's all working nicely.
Running postfix for yon e-mail.
The only issue I'm having is that of mapping users to domains for sending.
As it is, they have a virtual username for e-mail, looking like user.domain that they use to get their mail at user@domain.xxx.
How can I set it up so that when they send mail it appends the proper domain, instead of the fully qualified name of the server? (aka user@boxname.mydomain.net)

Any and all suggestions greatly appreciated.

Fedora Core2
Apache2
Postfix / imap using mailbox format
webmin/virtualmin/usermin
squirrelmail