not sure if this applies to you but I had this problem a while ago and it was all to do with domain names.
basically, my ISP only allows emails to be send to or from at least one @yourdomain.com address.
what I had done was set the admins address to be something outside the domain. I'll try and explain here.
forum is held on www.myserver.com
admins address was set to administrator@differentserver.com
whileever the replies were going to myname@myserver.com it was fine because myserver.com was in at least one address.
when I tried it with another user. lets say user@theirowndomain.com the ISP bounced it because it thought it was spam. IE neither the sender or the recipient was from myserver.com so it was treated as relaying spam.
Basically what I am saying is make sure the administrators address for the forum in question ends in the domain that the forum is hosted on. That should sort you out.
Hope this helps.
Cheers
Kev