2.1.3.2.9 Conversation Topic

To generate a Thread-Topic header, clients set the value of the PidTagConversationTopic property ([MS-OXOMSG] section 2.2.1.5). Clients SHOULD set this property value to the same value as the PidTagNormalizedSubject property ([MS-OXCMSG] section 2.2.1.10) has, with any subject prefix removed, as specified in section 2.2.3.2.6.1.

MIME writers copy the value of the PidTagConversationTopic property to the Thread-Topic header.