Here's a scheme I've been thinking about. It's an opportunistic PGP setup: 1) Whenever you want to send an email you look up the PGP certificate for the domain

Here's a scheme I've been thinking about. It's an opportunistic PGP setup: 1) Whenever you want to send an email you look up the PGP certificate for the domain (yes domain) you are sending it to in some central keyserver. If you find it pin that certificate for future use and sign and encrypt the email. If you don't find it just send a normal email, possibly still signing it. 2) For a sysadmin to start using the scheme just publish a public PGP key in the central keyserver and when you receive an email that's encrypted with your domain key decrypt it and deliver it to your user. This buys you a few things. For large hosting providers (say Gmail) you get a little better security than the current SSL between mailservers setup because it's harder to MitM and can be relayed between mailservers without security issues. You also get a PGP signature attesting that the email does indeed come from @gmail.com and isn't spoofed. Individual users that care can then use this scheme to get user-level encryption by having email in their own domain @myname.net. You could even outsource the @myname.net mail server to a normal POP/IMAP provider and publish the PGP key to the central server yourself. The provider then gets the encrypted mail and you decrypt it locally in your mail software. But maybe relaying doesn't really happen these days and fixing STARTTLS would give you most of this with SSL certificates instead.