To add to the above post, the headers have to be unencrypted, else they'd have no method to actually send the email. If it were encrypted, they'd have no idea what the address was, or who to bounce it to if the address doesn't exist.

Email headers contain a lot of information. It has the various email addresses, servers involved, ip addresses, time stamps, subject, priority, and things like that.

The body of the email is the only part that gets encrypted when encryption is in use.