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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.