This has some important and legitimate uses. The Problems I won't claim that the spam problem isn't a real one. I seem to get a relatively little amount of spam, usually not more than one or two messages a week unless some unscrupuluous person realizes that they can annoy the entire MIT community by sending to public mail lists as well as individuals.
Spammers can get email addresses from a number of places.
Given a list of addresses, either scanned oneself or purchased, one then either finds a spam-friendly ISP, such as UUNet, or gets a free throwaway email account from a service such as Hotmail. One finds an advertisement, and using their service, sends mail to every address they can possibly come up with. It doesn't matter if all or even most of the messages get through, just so long as a few people get the advertisement. They then send mail to these addresses. If I post using an address that I don't recieve mail at, and the spammer picks up that address and sends it mail, I won't get it.