Akismet (http://akismet.com/) is a collaborative spam filtering service.

The absolutely worst thing that can happen in your anti-spam solution is to block a good email and not let anybody know about it!  Anti-spam solutions should always generate an NDR such that a legitimate sender can know their message didn't get through. (Of course, we know many legitimate users don't read nor understand NDRs, so there's still an issue)  A really good anti-spam solution should not only generate an NDR, but that NDR should have an "escape clause" in it that gives that legitimate user a special way to get through the anti-spam solution, if they take some reasonable steps.

The more data you send with each comment check, the better chance Akismet has of avoiding missed spam and false positives.