1.10. There is no message threading in my web-archives
Threaded mail readers like Pipermail don't assemble threads based on the 'Subject:' of the discussion - They use two special message headers designed for this purpose: 'In-Reply-To:' and 'References:'.
If your archives are flat, then this may be because your Mail User Agent (MUA) isn't creating these headers when you reply to a message. This isn't a problem with modern MUAs, however. Also note that an over-enthusiastic mailserver somewhere in the chain between your MUA and mailman could be stripping them out. Lotus Notes/Domino, cc:Mail, and (some) Microsoft Exchange servers are notorious for doing this.
Note that you cannot fix this problem with Mailman. Once the message has left the client, or once this information has been stripped, you are toast. There is no general way to determine (with even just reasonable accuracy) which message is being replied to, and which other messages are being referenced.
This problem can only be fixed within the MUA or the mail server where the information is being stripped.
With these two headers and the right code to parse them, you can do a pretty good job of threading (see <http://www.jwz.org/doc/threading.html>. If either or both of these headers get stripped or the MUA fails to generate them on reply, there is nothing you can do to go into the body of a message and recover that data.
Upon a lot of testing with Outlook 2000, Outlook XP and Outlook 2003 I have started to discover that the real problem lies within Outlook itself and not (neccesarily) Exchange Server as previously believed (although, a combination of the two could easily be the issue also). It may also lie in Outlook's transport mechanisim when it sends the message to the Exchange Server.
When Outlook is in IMAP mode (even connected to an Exchange server), threading occurs fine and no headers are stripped.
To demonstrate this (for Windows users):
1. Go to Mail (located in Control Panel)
2. Add a new profile named 'imap'.
3. Create a new account in this profile using IMAP settings rather than using MS Exchange (If your Exchange server is configured to accept IMAP, that is).
4. On the Mail window, check the 'Prompt for a profile to be used' radiobutton.
5. Hit Ok, close Outlook (if it's open) and reopen it. It should prompt you for a profile to use. Select the 'imap' profile.
6. Send a test message to your list and then reply-to-all.
7. Voila! The threading works...
Why this is happening isn't clear yet. It seems to have something to do with Outlook trashing headers when composing the envelope that it sends to the Exchange Server. In IMAP mode, Outlook uses smtp for the transport.
As for an eta on a fix, I am currently discussing the matter with Microsoft and as soon as I have more information, I will be sure to post it here.
Converted from the Mailman FAQ Wizard
This is one of many Frequently Asked Questions.