By appending in the Posfix configuration file line smtpd_recipient_restrictions = ... spamcop and spamhaus , spam decreases in like 95% without even touching your server further (spamassassin I'm looking at you).
I you add greylisting you get rid virtually of all of spam.
I've never had a case of a false positive complain but yes, they can happen and I don't terribly mind because that means the sender's mail server is blacklisted and they should know.
Also in any case my recommendation is to use a 3rd party mail service, for deliverability reasons mostly.
Point taken, but I've been on the receiving end of what I would consider false-positive blocks by Spamhaus & co. They sometimes have policies about what's considered spam that I don't think all their end users would agree with. I've been blocked for having an IP address on the same provider as someone else who allegedly advertised their website via spam. If you're running a mailing list, dealing with the anti-spam stuff is at least as big a problem as the spam it was supposed to solve.
I don’t really like greylisting, as it takes longer for email to get through. What did help without any perceivable loss whatsoever is being extensively strict about SMTP specifications (e.g. proper hostnames in EHLO) and things like PTR records. I really like watching these ‘5xx: Client <something> rejected’ flying by in my mail.info :)
I'm all for cleaning up email.