In practice, Message-IDs are not necessarily unique. This means that it is neither secure nor reliable to discard a message with the same Message-ID as a previous message. If you want to discard duplicates, you should compute a cryptographic hash of each message. In theory the value of a References field is tokenizable , consisting of a series of words and message identifiers. Writers should follow the same format.
Writers use References to indicate that a message has a parent.
The last identifier in References identifies the parent. The first identifier in References identifies the first article in the same thread. There may be more identifiers in References, with grandparents preceding parents, etc. I recommend that writers support References as described above.