MTA-STS Checker

MTA-STS (Mail Transfer Agent Strict Transport Security) forces TLS encryption on inbound email connections, preventing downgrade attacks. The Untangled Labs MTA-STS Checker validates the _mta-sts DNS TXT record, fetches the hosted policy file from the well-known URL, and checks the mode (enforce, testing, none), max-age, and MX hostname patterns. Free, no account required.