These are  probably encoded MIME  messages. The people who  designed MIME
made the  strategic decision, after a  long debate, that MIME  would be a
MUA-only (or MUA-mostly) issue and that no modification would be required
to MTA's for MIME, with the possible exception of gateways. This means it
is the user sending the message who decides whether or not recipients can
handle MIME. In the case of a  mailing list, it is very unlikely that all
subscribers will have MIME user interfaces,  so you have a choice between
not using MIME  and sending rubbish to some of  the subscribers. LISTSERV
can't  take the  responsibility of  decoding  the message  for the  user,
especially as it can only decode certain types of MIME messages. There is
little or nothing LISTSERV can do about audio tracks, multi-part messages
and  the like.  The  only case  which  is  simple to  handle  is a  plain
single-part text  message where  the contents have  been encoded  to pass
through some 7-bit mailer.
 
  Eric