Spamarrest: the spam filter that spams

I've been using spamarrest for years as a spamfilter. It's an extra POP mail server that sits between you (meaning your mail client: outlook, thunderbird, etc.) and your actual email POP server. If a sender is in your whitelist, it gets through. If not, the sender gets a bounce message with a captcha challenge that will let them add themselves to the whitelist. You can override senders into a blacklist too.

The pros:
You don't have to scan email to recognize spam. You don't even download the spam at all.
Real people can still get through even if you don't have them in your whitelist.
Spam is reduced by 100% - not 98% (as you may get with Bayesian or advanced filters)

The cons:
Asking others to help you manage your whitelist is obnoxious.
Spamarrest bounces on every non-whitelist & non-blacklist email - that means they are increasing global email traffic by a 1:1 ration of spams received by their customers.
Spamarrest emails your new whitelist contacts with advertisements - i.e., SPAM. I think it's a no-brainer that spam-filtering companies should avoid spamming.

They do have a feature where you can use them for SMTP too, and every outgoing message you send can automatically add the recipient's email address to your whitelist. That ensures your new contacts won't get a bounce or a spam, and it doesn't take any work on your part.