I had a thought today when going through my email. The perfect way to pass messages undetected would be to pass them in a form that people normally delete and disregard out of habit. 

Spam! Pass messages by encoding them in spam. Spam already has random artifacts in it, letters altered from what they would normally be. All you have to do is find a technique to modulate those artifacts and interpret them on the other side. Add to that a winnowing technique to determine which ones are just chaff and which ones actual messages…