We use Postfix with a bunch of the *_restrictions options set, along with a handful of RBL checks.
We also use amavisd-new to integrate clamav, spamassassin (with a whole shwack-load of extra plugins), and dspam.
We block about 2 million messages a month using the above, let through about 20 thousand "possible spam" messages, and about 30 thousand clean messages.
For a school district with 50 domains, 1600 staff accounts, and 7500 student accounts.
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