On a legal standpoint it's difficult to say (not a lawyer)
however in the US a private business does have the legal right to do what they want with the e-mail so stretching it a bit I would have to believe that legally you own it and can do as you please.
Your forums are not public. They are something that you offer to the public for free most likely. But they are not owned by the public. you have the legal right to do anything you want with the information posted to your site.
Now also anyone can sue you for anything. But you are God in your forums and if you want to be all knowing about what your members are doing and saying while at your site. It's your right.
Ethics should come into play. But then again ethics should also be at play with the spammers and scammers.