I'd be interested in learning how well the mailhandler/listhandler combo
works for you as traffic increases. If you run your own server you
should probably find a solution that is more tightly integrated with the
MTA if you expect hight traffic lists.
      
Yes - I've thought about this a bit. But the thing to remember in this
particular case is that outgoing mailings are handled by ezmlm.
    

Yes, I know. I am the author of listhandler.module. :o)
ooops :).
  
Mailhandler sends a single email to the list and the ezmlm takes over.
    

Listhandler sends the mail, IIRC.
ahhh thanks.  I haven't had to look at the code much because everything works :).  We are getting into dependency hell however - Mailhandler/Listhandler/ForumMail - it would be nice to consolidate.
  
This doesn't directly address the larger issue you raise - but it does
remove the responsiblity for the mail details to a proven tool that
scales fairly well.
    

There shouldn't be a problem at the sending end. I was rather thinking
that the retrieving end could get clogged up if there are a lot of mails
sitting in the inbox. The worst thing that could probably happen is that
threading gets mixed up, but one never knows.
yes - I think i've seen this over on CS with the Forummail/listhandler
IOW, there should be a mechanism to check threading once in a while and
fix it if for example parent mails were received after their children.
yeah - not sure how this works currently.  Maybe as a mail is processed and attaches itself to a thread it checks to see if things are in order?
  
We've introduced, at least conceptually, a separation between the
mail and the mailhandler - it should be trivial to have the mail
handled in different ways. (as long as the underlying mechanism
supports subscribe, unsubscribe, and is a member of type commands.
    

Matt and I once tried to write a module that allows subscribing etc to
mailman lists through Drupal. It didn't get further than a proof of
concept, but you might want to revive it. Beware: It relies on
Snoopy.inc to do some screenscraping. You can find it in the attic of
the contrib/nodules cvs (mailhandler_api).
I actually took a look at this - as well as an ezmlm module someone had started.  One of the nice things about ezmlm is that there is a complete command line interface which we've used.