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…

OPPORTUNISTICTLS is not a valid SMTP command - opportunistic TLS describes a connection behavior (attempt TLS but fall back to plaintext if unavailable) rather than a defined protocol command.

UPGRADETLS is not a recognized SMTP command or extension defined in any RFC or mail protocol specification.

FORCELTS is not a valid SMTP command and appears to be a misspelled distractor with no basis in any mail protocol specification.

STARTTLS is a standard SMTP extension defined in RFC 3207 that signals to the receiving mail server that the client wants to upgrade the current plaintext session to TLS encryption. Once both servers complete the TLS handshake, all subsequent SMTP communication is encrypted, protecting message content from interception in transit.