For Postfix v2.6 or greater, any of the "milter" software listed above can be used.
Another way to integrate postfix and spamassassin is to use spamd. See IntegratedSpamdInPostfix. This method supports after-queue filtering only and is not quite as robust or nearly as efficient.
IntegratePostfixViaSpampd is a more robust and efficient solution, but also a more complex one. It supports before-queue or after-queue filtering.
IntegratedInPostfixWithAmavis is more complicated still but provides a way to plug in ClamAV virus scanning as well. It supports before-queue or after-queue filtering.

Integrated into Qmail

Use SpamAssassin to reject spam at SMTP time with Simscan. Simscan works natively with Netqmail-1.05, or with vanilla qmail-1.03 with the qmailqueue patch.

...

As of Exim 4.50, by compiling Exim using WITH_CONTENT_SCAN=yes (see details, more). As an Exim 4.4x Exiscan-extended ACL condition stack, Exim can reject spam after reading the body, but before Exim acknowledges acceptance of the email. Debian Linux's exim4-daemon-heavy comes with the exiscan patch built in. You can discover its presence in your distro's exim binary directly with the command exim -bV or exim4 -bV; look for the phrase Contains exiscan-acl patch.
As an Exim 4.x loadable module or local_scan.c replacement. SA-Exim also allows SpamAssassin to reject spam before its accepted by your MTA. More info at http://www.timj.co.uk/linux/exim.php. On debian this is the default method: install the sa-exim package and edit /etc/exim4/sa-exim.conf to enable it.
As an Exim transport.

Integrated into Courier-MTA

...

...