How about generating an automatic report to postmaster at the offending sites
(making sure it is under 50,000 characters), each time that it occurs. Also,
be sure to change the text and characteristics of the report to defeat any
attempts at automatic filtering out the reports at the offender's end.
This is only 1/2 ;-). I have had incredible amounts of my time wasted by
mailers with piddly byte limits. CompuServe is one of the worst offenders;
not only do they do this but they also reject mail when a user has too many
unread messages. Apparently they have never heard the term `denial of service
attack.'
More seriously, I think it's probably alright just to expell people from the
list whose mailboxes bounce, and send them a nice letter to the effect that
they're welcome back on the list when they get access to a machine which
supports e-mail.