Suggestions for a secure mailserver setup for a small group

Hi

I am in charge of setting up a server for our group. The server in question is supposed to act as a Samba server for windows XP professional hosts, an occasional server for intensive fortran matlab calculations. The server has hardware RAID 5, intel pentium 4 - 3.6 GHz, with 897MB of RAM (as judged from the size ofproc-kcore), and plenty of hard disk space. I have an anticipated user base of about 10-15 users, each of whom will need IMAP access to their email and ability to send email.

Years ago, I played a little bit with sendmail, and from what I remember, I got some stuff going, but it was not easy. I need to set up things so that is server.domain.yyy. I vaguely remember that such a thing used to be possible by editing sendmail.cf or some such thing.

I have set the size of thevar parbreastion (ext3) to be about 30G in size. What kind of imap server should I set up (its a medium sized group, so if something lightweight would do, it might be a better idea). Further, what ports would I need to open ? IMAPS and SMTP ? I get the feeling that smtp is a bad idea.

Any links to example configurations etc. would be welcome.

Thanks.