My understanding is that TLS is an encryption technique that allowing two STMP servers to communicate with each other securely. If HTTPS is used to connect to an STMP serve is that the same as using S/MIME?
Answer
No. TLS encrypts the communication channel. S/MIME encrypts the message. I.e., it's the difference between "talking openly on a secure line" and "talking in code on an insecure line."