C# EDSM ADSI Provider ARS Search based on edsva-MsExch-ActiveMailboxServerName

Can we use EDSM ADSI provider to search based on the built-in custom Exchange attribute "edsva-MsExch-ActiveMailboxServerName" ?

The code below return 0 result even though there are a lot users has value of the attribute "edsva-MsExch-ActiveMailboxServerName"

If we change the search filter to another attribute "sn", it will work

filter.Append("(&(sn=" + sSN + "))"); 

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          const AuthenticationTypes ADS_EDMSERVER_BIND = (AuthenticationTypes)32678;

            DirectoryEntry strDefaultNamingContext = new DirectoryEntry("EDMS://servername/DC=test,DC=com");

            strDefaultNamingContext.AuthenticationType = ADS_EDMSERVER_BIND;

            strDefaultNamingContext.Username = @"NIH\" + userID;

            strDefaultNamingContext.Password = userPW;

            // DirectorySearcher

            DirectorySearcher ConfigSearcher = new DirectorySearcher(strDefaultNamingContext);

            ConfigSearcher.SearchRoot = strDefaultNamingContext;

            StringBuilder filter = new StringBuilder();           

            filter.Append("(&(edsva-MsExch-ActiveMailboxServerName=" + sExServer + "))");          

            ConfigSearcher.Filter = filter.ToString();

            ConfigSearcher.PropertiesToLoad.Add("edsva-msexch-activemailboxservername");

            ConfigSearcher.PropertiesToLoad.Add("sn");

            ConfigSearcher.SearchScope = SearchScope.Subtree;

            SearchResultCollection results = ConfigSearcher.FindAll();

            foreach (SearchResult result in results)

            {

                s = result.Properties["sn"][0].ToString();

                Response.Write(s);

            }

            //cleanUp

            strDefaultNamingContext.Close();

            strDefaultNamingContext.Dispose();

            ConfigSearcher.Dispose();

            results.Dispose();