While we're all ranting (SPF vs. YASAF, etc.)... has anyone seen an open source equivalent of this system:

All messages from unknown senders are bounced. The bounce contains a link to a robot-unfriendly "prove you are a human with a real email address page" that consists of one of those "enter the letters you see in this picture" forms. Doing that validates the sender as a real person, not a spammer, and eliminates the need to run something like spamassassin.

A database is kept of all valid senders, and whitelists (discussion lists, etc.) to make this a one time requirement per recipient (or optionally per recipient domain, per server etc.)

I'm thinking something like this could be added to courier as a filter - much like your filter concept Gord - just wanted to solicit opinions about it - the company I saw doing it was offering it as a mail filter service (set your MX to them, and tell them where to send the cleaned mail to IIRC) neat in a way...

Thoughts?

Thanks!

m/