I'm subscribed to some mailing lists and every day people reply to a wrong mail (or they don't reply at all), so that their mail lands in the wrong (or a new) thread.

I set the mail display in "View ? Sort by" to "Threaded".

Example: mailing list "Foobar":

[Foobar] random topic
  Re: [Foobar] random topic
  Re: [Foobar] random topic
    Re: [Foobar] random topic
  Re: [Foobar] random topic
[Foobar] I'm John Doe
  Re: [Foobar] I'm John Doe
  Re: [Foobar] Welcome, John
Re: [Foobar] random topic

There are two discussions, one about "random topic", one about "John Doe". The subject line changes in the discussion about John Doe, which is fine (no problem here). But the last mail should be pigeonholed in the first thread. Instead it is at the top-level.

Now, how can I move that last mail into the correct thread? I tried to drag&drop it at the mail I think it should be a reply to, but this doesn't work.

I think theoretically it should be possible by fiddling with the mail headers after receiving the mail, but this doesn't seem to be a comfortable way.