Integrated into Sendmail
With Sendmail, Spamassassin can be called for every user by editing /etc/procmailrc . More details at http://www.stearns.org/doc/spamassassin-setup.current.html#sitewide
Integrated into Postfix
Integrated into Qmail
Can be integrated via qmail-scanner, an add-on to qmail. Info available at http://qmail-scanner.sourceforge.net/