aarrgh. SMTP is already complex enough. the last thing I want to see
is more complex MTAs adding more failure cases.
I was thinking in terms of an ESMTP extension. In any event, by your own
admission, doing it as a MIME type would break a lot of software at the end
points. At least this way it is only attempted between mutually consenting
MTA's and is transparent to the end users.
an MTA that transparently corrupts your mail isn't necessarily a good thing.
note also that compression algorithms tend to be specific to particular
kinds of content - what works well with text or object code typically
doesn't work well with images, audio, or video.