If you want to use LDAP/System.DirectoryServices directly, I wrote a whole
book about that stuff that might be useful to you (see link in the
signature).  There's a free chapter and code samples available for download
if you want to try before you buy.  :)

Essentially, if you have the SID of the user, you can create a binding
string for the DirectoryEntry like this:

LDAP://<SID=S-1-5-20-xxxxx>

Where the SID value shown there is the value you get from calling ToString
on the SecurityIdentifier object.  Once you have that, then getting the
user's email is easy:

string mail = (string) entry.Properties["mail"].Value;

The main trick may be figuring out how to set up the security context
properly in ASP.NET to access the directory.  This is covered in detail in
ch 8, and I've posted on numerous approaches to this problem in these
newsgroups over the years the Google will find for you.

Best of luck,

Joe K.