I would be remiss if I didn't mention that one of my colleague's here at
Google thinks this is all silly, and we should instead figure out how to
not base64 encode the messages in the first place, but I think that's a
higher barrier than this.
The good news is that we figured that out in 1995 in RFC 1830, very
slightly updated in 2000 in RFC 3030. But the bad news, as you are surely
aware, is that it's been 20 years, nobody uses it, and since it'd require
changes to all of the MUAs that create and display MIME messages, it's
unlikely to happen.
Given that the bloat from base64 is only 25%, how much real benefit is
there likely to be from compression? I know that people mail around giant
Powerpoint and Autocad files, but I have no idea how compressible they
are. Recent MS office formats pptx, xlsx, and so forth are actually zip
files, so they're not going to compress at all beyond what you get from
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