SpamAssassin

We're using the default spamassassin setup. No fancy custom filters, no frontends, just a reference in /etc/postfix/master.cf and that's it. If spam becomes an issue, we can have users give us some spam messages to train with, which is just a command or two away, found in the spamassassin documentation.
Spamassassin is currently quietly discarding messages marked as SPAM. We shouldn't have any issues with this, because our filters are set somewhat conservatively.