Well, folks, I tracked the cause down, and am about to ask that the
sitation be fixed.  (CC'ed it to Jory.)

  Fortunately this was only affecting a few people;  If you were one of
the few who received these "No such user: {something} @ winnt.mail", I
predict that the reason you received such an error message was that you
sent a message to the PICList with parentheses () in the subject line,
and that the error you received back had all or part of what you typed
in that subject line, as the e-mail address that was a non-existent user
at winnt.mail - I couldn't get anyone to send full headers my way, but
once I tested this scenario, I got full headers myself, so I can write a
note to the source of the problem.

  (In plainer english, if your subject line was "[OT] Weather!
(foobar)", you would have received an error back that said
"foobar@winnt.mail - no such user here.", in the body, with "No such
user!" as a subject line.)

  Probably just a broken internal mailing Daemon or something, I'll let
the person talk with their ISP about fixing it!

  Mark