What is MTA-STS and how does it secure email in transit?

MTA STS secures messages during transit between sending and receiving mail servers. It does not secure content at rest. It enforces:

• A TLS encrypted connection.
• A trusted certificate issued by a recognized authority.
• A policy that tells other servers exactly how to interact securely.

If a server cannot meet these conditions the message is not delivered. It focuses entirely on server to server SMTP transport and does not modify how users themselves access their mailbox.