I have one comment to make on our ability to determine whether the e-mail
of the future is ever recursively-encoded.
We can outlaw it, but we are building a spec which allows one to put
an arbitrary file into a part of a message. People will do this to
mail files. Gateways will be programmed to do it too.
We can outlaw it but it will happen just the same. A lot.
Some UAs will be able to handle it smoothly. Others will not.
Perhaps if someone were to generate effective propaganda
(e.g. UA & gateway implementation guides that developers actually
acquire and follow) the problem can be reduced somewhat.