Thomas Ward via Mailman-users writes:
Any chance Thread-Topic or such is spawned by the msgdata object instead of directly in the message?
No, the string "thread-topic" does not appear *anywhere* in the Mailman core sources. Mailman itself has no concept of threads. HyperKitty uses References, Reply-To, and Subject to algorithmically identify threads. Mailman does have an independent concept called "topic", which can best be thought of as "sub-list". But it uses the X-Topics header, and as far as I know doesn't pay attention to threading.
AFAIK, Thread-Topic is a Microsoft-specific device. If you use Exchange or Outlook, you have to negotiate with that software about the use of Thread-Topic. My guess is that it is generated by Exchange because somebody at Microsoft decided that threading should be enforced by the server, not inferred by the MUA.
Unfortunately there's very little documentation on msgdata in the handlers and what it is/contains so I'm asking again.
I guess the relevant documentation is RFC 5321. ;-) msgdata's original purpose is to contain envelope and delivery information.
That is, any data about the message that the LMTP runner knows that is not in the message header itself. HELO hostname, sender, recipients, IP address of SMTP client, timestamp of receipt is what I can think of offhand. There may be other stuff in there that is parsed out of the message and copied to msgdata for convenience but I don't know of any offhand. I'd have to read source to confirm.
Due to the holidays I didn't get around to testing/poking this so...
You're welcome to ask even if you haven't. Of course we prefer that you investigate as much as possible, but we're all grownups. Everybody has their limits, and we cover for each other as we can.
Steve