MTA-STS (Mail Transfer Agent Strict Transport Security) is a security protocol designed to improve the security of email communication by enforcing the use of TLS (Transport Layer Security) to encrypt email traffic between mail servers. It helps prevent man-in-the-middle attacks and downgrade attacks, where an attacker could intercept or tamper with email messages in transit.
See my blog post on how an MTA-STS policy works and how to implement it on GitHub pages with a step-by-step guide.