Simple Mail Transfer Protocol Secure (SMTPS)
SMTPS (Simple Mail Transfer Protocol Secure) refers to a method for securing SMTP with transport layer security. It is intended to provide authentication of the communication partners, as well as data integrity and confidentiality.
SMTPS is not a proprietary protocol and not an extension of SMTP. It is just a way to secure SMTP at the transport layer.
SMTPS With REVLY
This means that the client and server speak normal SMTP at the application layer, but the connection is secured by SSL or TLS. This happens when the connection is established before any mail data has been exchanged. Since whether or not to use SSL or TLS is not negotiated by the peers, SMTPS services are usually reachable on a dedicated port of their own.