MTA-STS
MTA-STS (Mail Transfer Agent Strict Transport Security) lets a domain publish a policy, via a DNS TXT record plus an HTTPS-hosted policy file, declaring that mail destined for it must be delivered over TLS with a valid, matching certificate -- and specifying whether a connection that can't meet that bar should be deferred or rejected. Without MTA-STS, email between two mail servers that both support TLS is usually encrypted opportunistically, but an attacker in the network path can often force a downgrade to an unencrypted connection without either side noticing. MTA-STS closes that gap by making encryption a declared, enforceable requirement. It's defined in RFC 8461.
By AdminKit team, Netbase JSC · Updated