Mail Transfer Agent-Strict Transport Security (MTA-STS) is a relatively new standard that enables mail service providers the ability to enforce Transport Layer Security (TLS) to secure SMTP connections. It has been proven to successfully mitigate TLS downgrade attacks and Man-in-the-Middle (MitM) attacks. In September 2018, the standard was first documented in RFC 8460 and was published by Google in September 2018. The primary goal is to improve transport-level security during SMTP communication, ensuring the privacy of email traffic.
Source: https://thehackernews.com/2021/01/enhancing-email-security-with-mta-sts.html