I have written a lengthy explanation here http://www.proudprogrammer.no/web/ppblog.nsf/d6plinks/GANI-A6YFK5.
Very shortly; Prompt the user when he or she attempt to click on a mail link containing a redirect-URL. These URLs uses the at-sign to redirect to other sites. The format is like this:
http:// <user info> @ < the real ...