This is one of Doug Otis's favorite rants, but I'll try to explain it in fewer than 54 paragraphs. *grin*

Today, all of our spam / not spam decisions are based on one or more of the following criteria:

• source IP address
• guesswork
• content comparison

Most of the time, any decisions made about the trustworthiness of the source IP address are based on guesswork too, but there just simply isn't anything else we can rely on about a message.

I have a somewhat unique perspective, in that I've done anti- spam work for the world's two largest free web-based e-mail providers in the world...and I gotta tell you, rate limits can only ever be a small part of the picture. We could drop it down to one message per day, and the spammers would simply register more accounts so they can send out the same amount of crap.

The really important part -- for any free service, not just e-mail -- is registration controls. It's amazing how many sites don't bother to protect that route until it's too late; instead, they're all excited because they're so popular! All those new accounts...except, why are they all 21-year-old men named John who live in Korea and only ever access the "compose new message" page?