It should also be mentioned what happens if a sender does space-stuffing
and the receiver doesn't know to undo space-stuffing - which would be
typical for all MUAs that exist today which don't understand
Format=Flowed. Such text might best belong in the Failure Modes section
(which might be renamed "Interoperability Issues" perhaps). I don't have
any text to recommend for this, though.
I am in any case curious as to why space was chosen for the stuffing
character. I would have expected `\`, which is more usual in such
situations, and would make it immediately apparent to a user using an
unmodified reader that some special convention was being used.
The answer is quite simple: You can see \, whereas space is invisible.
If we learned nothing else from quoted-printable, it is that people will
not deploy mechanisms that make their text look ugly. And \ definitely
qualifies.
Ned