This should setup your mail server to block all incomming spam while still letting users have easy access to e-mails that may have been legitimate. 

Name: Dovecot
HomePage: http://dovecot.org/
Function: Dovecot is an open source IMAP and POP3 server for Linux/UNIX-like systems, written with security primarily in mind. 

Name: Postfix
HomePage: http://www.postfix.org/
Function:  Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users

Name: Squirrel Mail
Homepage: http://www.squirrelmail.org/
Function: SpamAssassin is a mature, widely-deployed open source project that serves as a mail filter to identify Spam 

Name: Spamassassin
HomePage: http://spamassassin.apache.org/
Function: SpamAssassin is a mature, widely-deployed open source project that serves as a mail filter to identify Spam 

Name: Procmail
HomePage: http://www.procmail.org/
Function: procmail is a set of rules for each mail that comes into the system.