STARTTLS
An SMTP extension that upgrades a plaintext mail connection to TLS encryption mid-session.
STARTTLS is the SMTP extension that lets two mail servers begin a connection in plaintext on port 25 (or submission port 587), then upgrade to a TLS-encrypted channel using the STARTTLS command. Because the upgrade is opportunistic, an on-path attacker can strip the STARTTLS announcement and force the session to stay in plaintext, which is why MTA-STS and DANE exist: they let receiving domains require TLS rather than merely offer it. STARTTLS is also used by IMAP, POP3, and other protocols that started life unencrypted.