Since the emails that your users *are* getting, i.e. the command
response saying "blah blah has been sent to you under separate cover"
is administrative mail that would have the null MAIL FROM:, I don't
think the problem in this case would be that msn.com is rejecting
MAIL FROM:<>.

I sent a GET CURRENT SCLISTS to the Sierra Club LISTSERV, and the
file returned was 31K. Could it be that MSN has put a limit on the
*size* of messages it accepts? Does this seem to happen only with
larger messages? Are you able to send that file to the person
complaining from your own address?

Francoise