C# Active Directory: Get domain name of user?

I know that this type of question has been asked before, but other methods are failing me right now.

As it stands our windows service polls AD, given an LDAP (i.e. LDAP://10.32.16.80) and a list of usergroups within that AD server to search for. It retrieves all users within those given groups, recursively searching those groups for more groups as well. Each user is then added to another applications authenticated users list.

This part of the application is running successfully. However, we're in need of each user's friendly domain name (i.e. the part of their login DOMAIN/username)

So if there is a user that is part of TEST domain, named Steve: TEST/steve is his login. I'm able to find steve in the AD, however I also need "TEST" to be stored along with his AD information.

Again, I can find 'steve' fine by using a directory searcher and the LDAP IP I'm given, but given the LDAP IP, how can I find the friendly domain name?

When I try the following code I'm given an error when attempting to access the 'defaultNamingContext':

System.Runtime.InteropServices.COMException (0x8007202A): The authentication mechanism is unknown.

Here is the code:

    private string SetCurrentDomain(string server)
    {
        string result = string.Empty;
        try
        {
            logger.Debug("'SetCurrentDomain'; Instantiating rootDSE LDAP");
            DirectoryEntry ldapRoot = new DirectoryEntry(server + "/rootDSE", username, password);
            logger.Debug("'SetCurrentDomain'; Successfully instantiated rootDSE LDAP");

            logger.Debug("Attempting to retrieve 'defaultNamingContext'...");
            string domain = (string)ldapRoot.Properties["defaultNamingContext"][0]; //THIS IS WHERE I HIT THE COMEXCEPTION
            logger.Debug("Retrieved 'defaultNamingContext': " + domain);
            if (!domain.IsEmpty())
            {

                logger.Debug("'SetCurrentDomain'; Instantiating