Keith didn't mention the other option, of which he is also aware, which
is to add the special "_" rule to the MIME quoted-printable spec. This
means primarily that underscores will have to be quoted in bodies.
Then, in a quoted-printable body, you'd replace every occurrence of _
with =5f. In the worse case, a dividing line like
________________________________________________________________
would become
=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=
=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=
=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f=5f
(This assumes that the rule permitting the use of underscores for spaces
is OPTIONAL in bodies, but required in headers. If it is required in
bodies, obviously the bodies become MUCH less readable.) So, anyway,
that's another alternative to the two Keith proposes. I'm not sure
which is best, frankly, but I can certainly live with any of them, and
have a mild preference for eliminating the _ rule entirely.