Besides the feature that is available in LISTSERV 1.8d BETA

* Language= NOHTML

consider line-limitations e.g.,

* Sizelim= 200

That particular value works well on one list that I operate, but YMMV.  I
suggest that you search your archives looking for the maximum size of
your average postings, plus at what point (in terms of number of lines)
your attachments to pop up.

Pete Weiss @ Penn State