this should get you started:

public static void LookupEmail(NTAccount account)
{
   using (DirectoryEntry user = LookupUser(account))
   {
       Console.WriteLine(user.Properties["mail"].Value);
   }
}

static DirectoryEntry LookupUser(NTAccount account)
{
   SecurityIdentifier sid = (SecurityIdentifier)
     account.Translate(typeof(SecurityIdentifier);

   string path = string.Format("LDAP://<SID={0}>", sid.Value);
   return new DirectoryEntry(path, null, null, SECURE_BINDING);
}

e.g.

LookupEmail(new NTAccount(WindowsIdentity.GetCurrent().Name));

-----
Dominick Baier (http://www.leastprivilege.com)

Developing More Secure Microsoft ASP.NET 2.0 Applications (http://www.microsoft.com/mspress/books/9989.asp)