UrlRewriting is one way to go:
http://weblogs.asp.net/scottgu/archive/2007/02/26/tip-trick-url-rewriting-with-asp-net.aspxOr design your own webservice say hosted on
www.shorturl.com.
1: When you send email....you store the redirecting url inside a table with key= the userid or may be something unique.
2: So when user clicks the link grab the key, get redirecturl and redirect them.