I've been running my own spam filter for many years now based on this super-simple heuristic: My filter looks at my outgoing mail, and any mail received from an address I've sent mail to, or with a subject that has appeared in my outgoing mail (possibly with a "re:" prefix) is marked as non-spam. Everything else goes in spam, and any spam message from an address I've never received mail from before is marked as unread. I get hundreds of spams per day, but only about a dozen from new addresses. It takes me about ten seconds to scan them for non-spam cold calls, which are extremely rare. The other source of false positives is things like subscription confirmations, but because I know to expect those, they are always at the top of the spam folder.
I put this initial system in place expecting to have to augment it later with a more traditional content-based filter, but this simple heuristic works so well I've never felt the need to implement that additional step.
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