I've heard of a method for semi-secure communication that uses any public email service like Hotmail, Yaoo, etc.
Basically, person "A" gets an account and writes his message, then saves it as a 'draft' copy.
Person "B" logs in and reads the 'draft', then adds a reply and re-saves it as a 'draft'.
The message is never sent so it doesn't appear in normal email message traffic, but both parties can read and reply as much as they like.