The answer to this is at the back of my mind, we had this happen a long time ago.....first though, are you receiving any notifications at all, for example, when someone reports a thread? How about Tom, receiving any notifications?
I do remember an incidence a while back where the BW server was blacklisted by an isp as too many people had used a "delete & block" or "junk" option when receiving emails, instead of logging in to change their options. If too many people send our emails to junk or block them, then the isp's will take notice.