I do this and I'm not sure how well it works.. What I do is this..

Main Site http://www.autographdealer.com
Mail site http://www.autographmail.com

But I think my problem in doing this may be that Both site are on the SAME server, so If you are thinking about this and don't want to get your main site looked at as a Spam site, you may want to make sure that it's on a different server.

I think Now that I have answered your questions I may go do this myself :)

I'm very interested to see what others have to say on the matter.

Greg