At sites like craiglist, it's against their terms of use (and they mention something about it being illegal or them taking you to court) to use autoresponders advertising stuff when someone replies to your classified post, so why the hell do all these people still do it? And how can it be illegal anyway?
I love how so many sites use the word "illegal." Like the cops are going to come over and slap the handcuffs on your ass. That's some funny shit.
While autoresponders themselves are not illegal, you may be conducting yourself in a manner that violates spamming convention online which may be enough to stand in court. I think they're really targetting the macro-autoresponders that do it on a massive scale being widely disruptive.