where is the Message-ID?

[Steve Kleene <skdeb@syrano.acb.uc.edu>]

I post occasionally to the mplayer forum, which is archived at:

  http://lists.mplayerhq.hu/pipermail/mplayer-users/

All posts there are done by e-mail.  When I (or a responder) posts, it shows
up there pretty fast.  Much later, I get a digest of the last few posts by
e-mail.

I prefer to use sendmail.  When I post a reply (here for example), I preserve
threading as follows.  I construct the message with mailto, manually add the
References, and send it off with sendmail.

With the mplayer forum, there's a problem.  If I wait for the digest that
arrives by e-mail, I can see the Message-ID of the last post and add it under
References to preserve threading.  However, I see no way to find the
Message-ID at the online archive.  So I can either wait a long time, or I can
screw up the threading.

Clearly other posters there have figured this out.  How do they (or their
mail programs) get a hold of the Message-IDs?  (I would ask them, but they're
not thrilled with posts that aren't directly about mplayer.)

Thanks.