No MTA-STS policy
ATTENTIONWhat it means
Nothing tells sending servers that mail to example.com must be delivered over TLS. An attacker positioned on the network can strip the encryption, and the sender will deliver the message in the clear rather than fail.
How to fix it
Publish a TXT record at _mta-sts.example.com, v=STSv1; id=20260101000000, and serve a policy file at https://mta-sts.example.com/.well-known/mta-sts.txt listing your MX hosts. Start in testing mode.
Check your own domain
This page describes the condition. To find out whether your domain has it, run the MTASTS test or the full checkup, which examines every record at once.
Understand it properly
This page covers one condition. For the whole picture, MTA-STS, defined, or start at the Health Library.
Where this comes from
We judge this against RFC 8461 section 3.1. Every finding on this site cites the specification behind it so you can check the work rather than take our word for it.
Other MTASTS conditions
MTASTS_MISSING
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.