I just learned of an interesting way to send and receive emails with someone that makes the transmission almost impossible to detect. What you do is share the username and password to a Gmail account. Then you compose an email, but do not send it. That keeps the email in your drafts folder. The recipient then just logs in and checks out the latest draft message. They can then either edit that draft, or delete it and start a new draft. Sneaky huh?
I found out about this hack because General Petraeus of the US military used this technique to communicate with his mistriss. Sneaky devil. Terrorists have used this technique as well. Your email client never actually sends the email over the wire via email protocol. The message just sits on some server. The method is not foolproof. They figured out Petraeus' antics while investigating a crime. He was outed.
