EWS: How to get last item in conversation thread?

Hi,

I need to reply to the latest email item in conversation thread. I have ConversationId stored as a string and ConversationIndex stored as a Base64String. I try to search by ConversationID, but it finds nothing. What I'm doing wrong?

I'm using Exchange 2010 SP2 and EWS 2.1.

ExtendedPropertyDefinition ConversationIdProperty = new ExtendedPropertyDefinition(0x3013, MapiPropertyType.Binary);
ExtendedPropertyDefinition ConversationIndexProperty = new ExtendedPropertyDefinition(0x0071, MapiPropertyType.Binary);

ConversationIdci = newConversationId(conversationIdString);
SearchFiltersf = new SearchFilter.IsEqualTo(ConversationIdProperty,ci.UniqueId);
var view = new ItemView(512)
                    {
PropertySet = new PropertySet(BasePropertySet.FirstClassProperties,
ConversationIdProperty, ConversationIndexProperty)
                    };              

varitemsInbox = es.FindItems(WellKnownFolderName.Inbox,sf, view);

I also tried the FindConversation ofExchangeService, but get "FindConversation is only valid for Exchange Server version Exchange 2013 or later." error, although according to MSDN it is supported in 2010.

Please help!