[Mailman-Users] How does Mailman determine what thread does a given message belong to?

[Krylov Ivan]

Hi!

I'm running a mailing list with several webmail client users who chose 
to receive digests. How should they properly send replies, so they get 
in the proper thread?

Moderator of mailman-users at python.org suggested me to write a proper 
Subject:
> People can reply to the digest and just change the Subject: and edit
> out the irrelevant quotes;
But when I tried simulating this behaviour by writing to 
<mylistaddress>@<mylistdomain> with Subject: being "Re: [<my list name>] 
<The thread I wanted to reply to>", I ended up creating a new thread. 
This probably was a mistake, and Mailman is able to determine which 
thread to put reply to by combination of Subject: and In-Reply-To: 
headers, but I am not completely sure.

Also another user replied to the digest she received without changing 
the Subject: (which was also a mistake), and this also caused creating a 
new thread.

Can somebody please point me at the logic which Mailman uses to 
determine the target thread of a given message? Does it only look for 
In-Reply-To: header, or it uses some heuristics which can be used by 
digested webmail users?

Best regards,
Ivan.