Conventional spam filters try to detect junk mail using a variety of techniques. They'll look at the sending address, perhaps the email server, check the subject and body for key words, maybe compare the message with a database of known spam emails. Yet it's still all very hit and miss: some spam will always get through, and genuine emails will occasionally be blocked.
Comodo AntiSpam does things differently. If someone you don't know sends you an email, then you won't see it at first: the program steps in first, and sends them a message. They'll be asked to type in a password that's displayed in an image, and reply with the result. A real person can do this easily, and as soon as the reply is received then you'll see their original email (and all other messages from them, in future). Spambots have no way of replying, though, so junk mail is automatically filtered out.
Comodo AntiSpam works with just about every POP3/ SMTP email client. And it imports your email address book, too, so your current friends and contacts won't have to go through this challenge/ response system, even once: they'll automatically be added to the whitelist.
Anyone who tries to contact you subsequently will have to reply to the authentication process before their emails will get through, though, so they will have a little inconvenience. And occasionally legitimate senders may be caught up in this (a newsletter you want to receive suddenly comes from a new address; they receive an authentication email, but the address can't take incoming messages so it disappears).
Still, you can view emails waiting for authentication at any time, and authenticate the sender manually if you like, so these problems are relatively easy to solve. And they're arguably a small price to pay for a spam-free inbox.
Comodo AntiSpam's challenge/ response system is a drastic, and slightly inconvenient way to block spam, but it does work. If you're drowning in junk mail, this is well worth a try