Sendmail redirection of mail to subdomain

[fafadia]

I have four places connected through VSAT. I wish to install a Mail server with the facility that any user nereby the VSAT centre should be able to access his/her mail either by Local LAN or by dialing in to the server installed at the VSAT place. Howerver, my central Mail server should be redirect the mail for the user at that place to the respective subdomain immediately on receipt of the mail. I wish to have a comman mail address as username@domain.com. I don't want to specify subdomain in incoming mails as most of my users will be moving from one subdomain to another.

In short, what I am looking for an intelligent routing of mail among subdomains. I can have, if required, four or more subdomain for mail servers at all the VSAT places.