Migrating from win2000 server

Hello
We currently have a simple network setup with win2000 server running IMAP, POP3 download from ISP & SMTP relay. Our ISP also hosts our domain.
I’m trying to replace this setup with OS X server 10.4, however, I’ve hit a bit of a brick wall, and not sure whether I’m just looking in the wrong place, stuck in my old method of seting up the server.
SMTP relay fails in tiger because relay connection is refused from ISP. On win2000 we used a login verification to ISP.
I would like to keep my MX record with my ISP and have tiger do a scheduled download of all the POP accounts and deliver to the appropriate mail accounts on my server.

I’m guessing that I might have to do some configuration via the terminal as I just can’t seem to find any way of doing this from Server Admin.

Any help or pointers greatly appreciated.
Leroy