Not a postfix user myself, but I really like [<a href="http://www.lifewithqmail.com/lwq.html" target="_blank">qmail</a>].
Use the qmail-pop3d daemon + spamassassin and you should be pretty close to what you want (if I read it correctly). Configuration can be a little tricky if your linux distro is old, but once its setup, its solid.
You'll also have to change the MX record for the domains you want your mailserver to grab.
[Edit] I think I did read it wrong. If you want the linux mailserver to check your email accounts and stuff them into local accounts/queues, [<a href="http://catb.org/~esr/fetchmail/" target="_blank">fetchmail</a>] should do that for you