Contains a binary value that indicates the relative position of this message within a conversation thread.
A conversation thread represents a series of messages and replies. This property is usually implemented using concatenated time stamp values. Its use is optional, even if PR_CONVERSATION_TOPIC (PidTagConversationTopic) is set.
MAPI provides the ScCreateConversationIndex function to create or update a conversation index. The function takes the current index value as a counted byte array and returns the index value with a time stamp concatenated onto the end. A message representing a reply to another message should use ScCreateConversationIndex to update this property.
A message store provider has the option of assuring that PR_CONVERSATION_INDEX is always set on incoming or outgoing messages. It can do this by calling ScCreateConversationIndex, either with the existing value if this property is set or with NULL if it is not. This action should be taken before IMAPIProp::SaveChanges is called.
All messages that have the same value for PR_CONVERSATION_TOPIC can be sorted on this property to reveal the hierarchical relationship of the messages.