In general, I’ve subscribed to the belief that email should not be considered private, particularly in light of the ease with which email may be accessed, copied and distributed. Silent Circle has a good write-up of the challenges with trying to make sure that email is secure.
Email provides no means to secure the headers (routing information, and the envelope). The routing information, which is visible by looking at the headers of any email message, by design, is all unencrypted. Any server in the path between sender and recipient, can view any portion of the headers, as they are stored as plain text in the beginning of the message.
Other sources for your reading pleasure: