Here is a short list of applications we are going to use to build a mail server.
Postfix - Mail Transfer Agent
Courier - an IMAP/POP3 server
SpamAssassin - for filtering spam messages
ClamAV - antivirus
Amavisd-new - a glue that glues Postfix with SpamAssassin and ClamAV
Sqlgrey - for greylisting emails
MySQL - for storing information about users, domains and settings
Maildrop - for delivering mails to users mailboxes and quota support