PostItem.ConversationID Property (Outlook)
This property associates items with a conversation. These items and the conversation all have the same value in their ConversationID property.
This property corresponds with the MAPI property PidTagConversationId.
If the PostItem object is created in a version of Microsoft Outlook earlier than Microsoft Outlook 2010, or if Outlook is running in online mode against a version of Microsoft Exchange Server earlier than Microsoft Exchange Server 2010, this property returns the same value as the ConversationTopic property.