Just started reading a new book, (fiction), in which a method was stated to avoid having your email read, without having to encrypt it, (they used simple word code in the book).
Both parties use the same email account and only write drafts, ie.
Person A writes a draft and saves it.
Person B reads, deletes, and writes a new one.
Person A reads, deletes, etc, etc.
No email is actually sent between servers.
Interesting idea, don't know how feasible it would be, (barring unauthorised remote access to the email server).