I use the emails received for replies as a notification, not as a form of communication with members on the board. The emails I think are just to let you know, hey you have a PM, its waiting for you in your inbox when you want to take a look at it. I dont feel like I'm going out of my way to have to go to dnforum and check it.
I would assume that a large number of people used their email accounts to reply back to PM's or maybe even threads...and what MediaHound said, is that thats not the case... The emails get forwarded to the system email address, causing communication errors and might even ruin transactions. You may know better, but does the person you are doing business with know?
MediaHound, maybe some big red bold letters should be slapped on each email, saying "Do not reply to this email."