If an AD attribute appears in one LDAP tool, and not the other, what can be the cause?

I'm using the same userID, LDAP connection string to bind to an LDAP server and fetch information.

• Tool #1 exposes the msExchMailboxGuid consistently and reliably.

• Tool #2 (.NET) doesn't expose the msExchMailboxGuid. There are occasions where it's null, and other instances where it isn't.

(Note: I'm not looking for code /developer support)

       DirectoryEntry de = new DirectoryEntry();

        de.Path = "LDAP://ldap.exchange.serverdata.net:636/DC=exchange,DC=serverpod,DC=net";
        de.Password = "PASSWORD";
        de.Username = "[email protected]";
        de.AuthenticationType = AuthenticationTypes.Secure;

        DirectorySearcher ds = new DirectorySearcher();
        ds.PropertiesToLoad.Add("cn");
        ds.PropertiesToLoad.Add("mail");
        ds.PropertiesToLoad.Add("samAccountName");
        ds.PropertiesToLoad.Add("msExchMailboxGuid");
        ds.PropertiesToLoad.Add("proxyAddresses");
        ds.PropertiesToLoad.Add("targetAddress");

        ds.PageSize = 800;
        ds.Filter = "(&(objectCategory=person)([email protected]))"; 

        SearchResultCollection results = ds.FindAll();

This code is very simple, but when I substitute [email protected] with [email protected], the attribute is gone. (but is visible in other tools)

Is there any logical AD reason why an attribute would be missing under certain circumstances? What are those circumstances?

How can I debug this kind of LDAP query?