Yes, that happens a lot. It is either something un-parsable by LISTSERV,
or something less important. For a while, I have seen trouble with
'From's like:
 
From:  [log in to unmask]
 
Maybe that is the case again? If it is not their mailer causing the
problem, which probably is the case, I suggest those users to use some
other utility to prepare their messages (if they have something similar
to NOTE, or whatever).  -turgut