I think what you have a 2 variant:
1 put the form in mail whare you just make a request about readed mail.
2 do you know how people generate a images via php? same you a create a counter for images any and when mail opened in mail have a vimk on the image http://www.mysite.com/images/counterimage.php
But in both solution have one problem user may configure mail as do not get mail as HTML. I this situation you are no not have a varian to get some info from user.
The best variant: you are registered user in our forum. do you remember how you do it? Yes, you submmited a registration using a link.
P.S. This is not a code, It's just a logical propose.