I meant to write that it is *not* difficult to encrypt the messages on the
server (if you have control of it). I would imagine that most e-mail
providers encrypt communications between servers. No real reason not to.
I was talking about running your own server and using a client. The only
point is to decentralize, but if the government is tapping the major backbone
then it probably wouldn't matter. It's harder to coerce many entities instead
of just one.
About GPG, that's correct. It's also not perfect because there's metadata.
The subject field of e-mails cannot be encrypted and it is still possible to
tell who is communication with whom.