They probably are not outgoing only. It's just that these people are very smart when it comes to spoofing bogus addresses in the mail from and reply to fields.
But can you blame them? I mean, if you were spamming thousands of e-mail addresses, you wouldn't want to be receiving thousands of return e-mails would you??
(sorry, a little humor)
'97 Cobra - VT billet stroker shortblock, LLX4, TwEECer RT v1.30A9
'91 coupe - Incon Twin Turbo (gone, but not forgotten)