As someone else pointed out, POP and IMAP is where the users feel an
impact. I would argue that few people run SMTP servers on slow links, and
while most users at work are on fast links (typically ethernet), most
users at home or on the road are on slow links.
Thus, perhaps an IMAP (and POP) extension to allow on-the-fly compression
would give the biggest bang for the buck. It has an additional advantage
of not causing the interoperability problems with a new Content-Encoding.
Such an extension would be useful even after RESCAP and some kind of
Content-Encoding compression were being deployed and would provide a
benefit to users even if the sender didn't support RESCAP or the new
Content-Encoding compression.
-Dan Wing