Are the people in question set to MIME and DIGEST?  Try setting them to
NOMIME if so.  (MIME is the default for all new subscribers in 1.8c.)
The problem with MIME digests is that not all mail clients are capable
of dealing with them.  Normally if the user turns MIME off, the problem
is alleviated.

If the users in question aren't set to MIME and DIGEST, you may have to
look a little closer at how the remote system is handling the incoming
mail.  Is there a per-message size limit or something like that?  It is
highly doubtful that LISTSERV is sending empty-bodied messages.

Nathan