Ldap query returns null result when deployed - asp.net

Question

I'm using a very simple Ldap query in my asp.net mvc 2.0 site:
String ldapPath = ConfigReader.LdapPath;
String emailAddress = null;
try
{
DirectorySearcher search = new DirectorySearcher(ConfigReader.LdapPath);
search.Filter = String.Format("(&(objectClass=user)(objectCategory=person)(objectSid={0})) ", securityIdentifierValue);
// add the mail property to the list of props to retrieve
search.PropertiesToLoad.Add("mail");
var result = search.FindOne();
if (result == null)
{
throw new Exception("Ldap Query with filter:" + search.Filter.ToString() + " returned a null value (no match found)");
}
else
{
emailAddress = result.Properties["mail"][0].ToString();
}
}
catch (ArgumentOutOfRangeException aoorEx)
{
throw new Exception( "The query could not find an email for this user.");
}
catch (Exception ex)
{
//_log.Error(string.Format("======!!!!!! ERROR ERROR ERROR !!!!! in LdapLookupUtil.cs getEmailFromLdap Exception: {0}", ex));
throw ex;
}
return emailAddress;
It works fine on my localhost machine. It works fine when I run it in VS2010 on the server. It always returns a null result when deployed.
Here is my web.config:
Asp.Net Configuration option in Visual Studio.
A full list of settings and comments can be found in
machine.config.comments usually located in
\Windows\Microsoft.Net\Framework\v2.x\Config
-->
section enables configuration
of the security authentication mode used by
ASP.NET to identify an incoming user.
-->
<!--
-->
section enables configuration
of what to do if/when an unhandled error occurs
during the execution of a request. Specifically,
it enables developers to configure html error pages
to be displayed in place of a error stack trace.
-->
I'm running it under the default app pool.
Does anybody see the problem? This is driving me crazy!

Answer 1

OK, so I forgot to add that I have changed the user account running the Default App Pool to a user that has auth to run LDAP queryies.