Excuse my lack of knowledge in terms of mail encryption, but wouldn't the knowledge of communication be enough to warrant further investigation, even if the text itself is encrypted?
Well, yes... but then, by openly encrypting your traffic, you would be stating that you are a law-abiding, technically sophisticated citizen, willing to provide decryption keys to legitimated law enforcement officials with an appropriate order.
People with real secrets to hide ought to escalate to steganography + subliminal channel communications.
