SpamSieve $30 (approx £23)

c-command.com This enhanced junk mail filter opens automatically with Apple Mail (and many other apps like it). You can use the scores it assigns to messages to process supposed spam in different ways, rather than just moving everything to a junk folder – you might simply colour some as a prompt to offer the tool further training. Thankfully, you can also block senders. The utility works with all of your email accounts, so you might consider turning off any server-side junk mail filters in order to help SpamSieve better learn by ensuring it can inspect all messages you receive.