What is

What is MTA-STS

MTA-STS publishes a policy that tells sending mail servers to use TLS, and which hosts may receive the mail. The policy id is a TXT record at _mta-sts. The policy itself is HTTPS.

_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"

The policy file is https://mta-sts.example.com/.well-known/mta-sts.txt. mode is testing, enforce, or none. The TXT line above is the example in RFC 8461.