spam-filters on output

Spam filters are common nowadays. My free Yahoo mail account has one, which does a reasonable job, with about 95% accuracy, and a little bit of false positives here and there. I noticed one interesting fact, however. Most of the spam is sent from Yahoo, Hotmail, and other mass-free accounts.

So here's an idea. Add an output spam filter. I write a mail, and send it. The system spam-scans the mail and returns me a notice if it thinks it's a spam. The advantage here: no false positives - I'll never lose mail, I will receive a notice that it was considered spam, and will re-send it, removing the offending words. Practically, this should almost never happen to normal users, but it can severly hurt spammers.