This code snippet allows you to search Active Directory for a given email address and
returns you the coresponding user object.  

String email = testUser@dev.lan; 

DirectoryEntry LdapConnection = new DirectoryEntry(); 
DirectorySearcher search = new DirectorySearcher(LdapConnection); 
search.SearchScope = SearchScope.Subtree; search.Filter = "(proxyaddresses=smtp:" + email + ")"; 
SearchResult result = search.FindOne();  

if (result != null)  { 
DirectoryEntry deDirEnt = result.GetDirectoryEntry();  
Console.WriteLine("Address Found"); 
DirectoryEntry d = deDirEnt.SchemaEntry; 
}  else  { 
Console.WriteLine("Address not found!");
}