The problem most likely is that you are using the Default option of MIME and
the users can't handle the MIME encoded digests.

This is one of my pet peeves actually as I tend to feel that NOMIME should be
the default and users that understand what MIME is, does, and know their
mailers can handle it can change themselves over to MIME mode far easier than
the unwary list owners that continually get stumped after encountering this
problem.

I recommend adding NOMIME to your Default-Options line, changing your list over
to NOMIME and then notifying your list giving instructions as to how to reset
MIME mode for those who desire that.

(The above is merely my own opinion and not worth shooting me over)

Randy