Ask HN: Spam Filters

While going through my gmail inbox today, I marked something as spam and was wondering : When I mark something as spam, and a great deal of other users also mark the mail as spam (because it obviously is), does it disappear from people's mailbox who haven't seen the mail, thus improving the spam filter effectiveness for more people.

Looking around, it seems like spam filters are trained using only certain trusted users. Does this functionality already exist?

I don't know what gmail does, but what you describe can be done with spamassassin. The Horde webmail program has a way for users to submit false positives and false negatives to the spamassassin training command.