SpamSieve from C-Command Software has been the go-to spam filtering system for all of the popular Mac email clients, including Apple Mail, for some years now.
What separates SpamSieve from many of the spam filtering systems built into email clients is how simple it is to set up and use. It also works better than most built-in systems.
SpamSieve learns as it goes, but even the first time you use it, it will catch most of your spam. As you continue to use it, SpamSieve just gets better and better. Its whitelist and blacklist features ensure that the email you want to receive and the email you don't want to receive end up in the appropriate destination: your inbox or your spam box.
SpamSieve is $30.00. A demo is available.
See other software choices from Tom's Mac Software Picks.