Email is transmitted across the Internet using the Simple Mail Transport Protocol. SMTP does not encrypt email, leaving the information in the message vulnerable to being read by an unauthorized perso

There is no standard SMTP command named OPPORTUNISTICTLS for initiating TLS.

There is no standard SMTP command named UPGRADETLS for initiating TLS.

There is no standard SMTP command named FORCETLS for initiating TLS.

`STARTTLS` is the standard SMTP command used to signal to the receiving mail server that the sending server wishes to upgrade the current plain text connection to a secure, encrypted one using Transport Layer Security (TLS). This command initiates the negotiation process for establishing an encrypted channel, after which email data is transmitted securely.