MTA-STS (Mail Transfer Agent Strict Transport Security) is a new standard (defined in RFC8461) that aims to improve the security of SMTP by enabling domains to opt into a mode that requires authentication with valid public certificates and encryption (TLS). MTA-STS forces a TLS connection, preventing suppression of the STARTTLS upgrade, and defines what the MX records should be for a domain, therefore preventing DNS query interception to redirect to another MX record by a malicious party.