STARTTLS is an email protocol extension that begins as a plain SMTP, IMAP, or POP3 session and then upgrades that connection to TLS. In practice, it lets mail servers negotiate encryption after the connection starts, rather than requiring a separate encrypted port. When the upgrade succeeds, the traffic between servers or clients is protected from passive interception while it is in transit.
In cyber security, STARTTLS matters because it improves transport confidentiality without changing how email is stored or read at the endpoints. However, it does not provide end-to-end secrecy, and it can be weakened by downgrade attacks if an attacker blocks or strips the upgrade request. Defenders often pair it with policies such as MTA-STS or strict TLS configuration to reduce those risks. In real deployments, STARTTLS is a baseline control: useful for protecting delivery paths, but not enough on its own when the threat model requires strong protection of message content.