Not the contents of the mail yet - so perhaps it's not quite time to break out the PGP ubiquitously (and it's not a lot of use if you only use it for suspicious things, so it's pretty well got to be all or nothing). But what if you wanted to make your email flist just a little bit harder to automatically snoop and mine?
Well, it seems to me that the answer is in randomly generated throwaway webmail accounts. Swap contact details with a person (using the same system) for the first time - this involves picking and registering a new webmail address.
Each time you email them (or they, you), pick a new randomly generated address for both of you, and include the username and password of their new throwaway address in the email.1
With a bit of standard markup for including the new throwaway address and password in the email, this should be relatively easy to automate... Though it might involve typing a couple of capchas when you send a mail.
Anyone fancy starting on a mutt or sup plugin? Some kind of uber-greasemonkey script might do something cool too.... Bonus points if you can make it work nicely with your threading engine of choice ;)
1 Of course, if only suspicious people use it, and if the syntax for describing a new address is easy to mine for, then we're back where we started. So we may have to present a new randomly generated presentation syntax in each email too. But that's doable :) Especially if you do it in the middle of a markov chain generated signature, where it won't get on anyone's nerves,