I understand the tribulations of picking through MIME and HTML in a
nonsupportive mail reader.  But., my current version of MS Outlook
includes HTML and MIME in outgoing messages by default, these
features are preferable for most of my mail; I have to explicitly switch
to plaintext mode for math-fun and seqfan postings, and sometimes
I forget.  I can't select HTML or plaintext per recipient, consequently
slips will happen, and I can't be eternally apologetic.  If people can't
overlook the occasional slip on this account, I can easily go away.
 
At some point, I think math-fun and seqfan will have to acknowledge
that technology is moving on.  Despite my personal distaste for
Microsoft and its applications, the reality is that MS Outlook is now
the most commonly-used email application, that it supports HTML
and MIME, and that users rely heavily on the associated benefits, such
as formatted and colored text, images, backgrounds, links, attachments,
etc.  I appreciate that math-fun and seqfan like to be accomodating,
however, accomodating HTML-disabled users will increasingly disaffect
a growing majority of HTML-enabled users, and will deprive math-fun
and seqfan of some very useful technologies (when MathML browsers
become common, will you not take advantage?).
 
I understand wanting to accomodate HTML-disabled users, and the
desire to save on server space; and undercurrents of nostalgia,
UNIXphilia, and MSphobia all warm my heart.  It may also be that
math-fun and seqfan want to discourage a floodgate of membership,
considering some recent incidents of inconsiderate new users
disrupting our coffeehouse atmosphere.  Notwithstanding, my
considered opinion is that catering to the technologically lowest
common denominator will become increasingly costly to math-fun
and seqfan as technology advances.