what you will need for a basic set up is
fetchmail - this will login into the pop3 accounts. download the mail thats there and pass it onto ur MDA
sendmail - this will do all ur mail server tasts
mailscanner and spammassin installed along side ur sendmail for that nice virus scanning and chekcing of spam. i use these and i am very pleased with them. and its fast
virus checking for mailscanner could be somethink like f-prot they do a free one for home use. also camAV fully free and open source
serch google for the links.
do u have sendmail and fetchmail installed? what distro are using.