if I remember correctly qmail has a ldap intergration addon that could handle roaming users. But if all users are roaming then how do you get the mail to the right box? y1 on box2 goes to check his mail in box3, how does he get it? It sounds like you need an imap/smtp server that everyone can connect to from the lan, you might want to block imap from the internet though.
Look at qmail, imap server cyrus(spelled right?) and lock down the smtp/imap server.
marc
