Enhancing Email Security with MTA-STS and SMTP TLS Reporting
In 1982, when SMTP was first specified, it did not contain any mechanism for providing security at the transport level to secure communications between mail transfer agents.
Later, in 1999, the STARTTLS command was added to SMTP that in turn supported the encryption of emails in between the servers, providing the ability to convert a non-secure connection into a secure one that is encrypted
