S/MIME was designed both for message signature and encryption. It is known that some transport data need to be in cleartext because of course only the recipient has the key to decrypt a message - still the message "payload" is encrypted, and it is in the server storage as well.
Then the transport may happen over an encrypted channel to ensure confidentiality of the whole message - but unluckily now you can only protect that data from/to your mail client and your mail server - whatever happens outside your mail server is not under your control - the SMTP protocol really needs an update - there's a good chance no transport encryption will be used, and even it it is, there is no provision to check the certificates of the server you're talking to.
Biting the hand that feeds IT
