> Thanks for your response, but I verified the lists configuration and there
> are no message size limits (the field value is set to zero).
> 
> Not sure if this matters, but several other lists routinely receive over
> 500 messages a day, with the message sizes averaging approximately 450kb
> per message - however, those messages are plain ascii messages - not
> MIMEd - and are going through fine, without any problems.

What I've been doing to provide extra filtering, etc. is wrapping
the mailing lists with procmail.  Here's a February posting with
some examples:

   http://www.python.org/pipermail/mailman-users/1999-February/000617.html

A recipe to block messages that are too large could be

   # block messages larger than 10k
   # attributed to Philip Guenther, procmail mailing list
   :0
   * > 10240
   {
       # Bounce messages should generally be sent to the envelope
       # sender, not the header sender.
       :0
       * ! ^\/Return-Path:\/.+
       * ! ^\/From  *\/[^ ]+
       { ENV_SENDER = $MATCH }
       :0 E
       { # This message doesn't have an envelope line!
      # This really shouldn't happen, so consult your
      # psychiatrist, or sendmail.cf hacker.  We'll
      # just drop the message on the floor, as there
      # really isn't a better place to put it.
      HOST
       }
       :0
       | (echo "To: $ENV_SENDER"; \
          echo "Subject: Message Too Large: Rejected" ; \
          echo ""; cat - ) | $SENDMAIL $SENDMAILFLAGS -- "$ENV_SENDER"
   }

Or you could just dump them all:

   :0
   * > 10240
   /dev/null
 
Chris