What I would do (hell, it may even be happening)
Rather than (moronically) sending each other emails, I suspect terrorists, criminals (and spies) who have an aversion to being caught would simply:
1) Identify a *public* channel for communication(s). Maybe a couple of binary newsgroups
2) post in one an NZB of a media file (don't worry if it doesn't work. Thanks to the media providers dark war on copying, corrupted media files aren't significant) which while not encrypted, has your encrypted message hidden inside it.
3) The actual intended recipient of the file will not be immediately apparent
4) The recipient replies the same way. If the channel is *initially* secured, it can be used to switch newsgroups/posting handles at will.
5) Notice how nothing in the UK governments land-grab of data could (a) prevent (b) identify this.
To be honest, I wouldn't even bother encrypting the source message. There's so much shite spouted online anyway, there's no way you could determine anything in isolation.
But then if I were a "terrorist" and my aim was to kill, hurt and maim as many innocent people as possible with my own survival being unnecessary, there's plenty of things I could do RIGHT NOW that could take a dozen or so souls out without really trying. The lack of such incidents leads me to wonder quite how "threatening" the "terrorist threat" is ?