STARTTLS
STARTTLS is a command that upgrades an initially plain-text SMTP (or IMAP/POP3) connection to an encrypted TLS connection, if both the sending and receiving server support it. It's "opportunistic" encryption by default: if a connection attempt doesn't offer or accept STARTTLS, most mail servers will silently fall back to sending the message unencrypted rather than failing the delivery, which means an attacker able to intercept and manipulate the connection can sometimes strip the STARTTLS offer entirely (a downgrade attack) without either party noticing. MTA-STS exists specifically to close this gap by letting a domain declare that TLS is mandatory, not optional, for mail addressed to it.
By AdminKit team, Netbase JSC · Updated