_MobileItem.ConversationTopic Property
Office 2010
Returns a String (string in C#) that represents the topic of the conversation thread of the item. Read-only.
Assembly: Microsoft.Office.Interop.Outlook (in Microsoft.Office.Interop.Outlook.dll)
For the MobileItem object, this property always returns Nothing (null in C#) because the MobileItem object does not support conversations.