Practical reference
MTA-STS policy examples for testing and enforce mode
An MTA-STS policy is a short text file served over HTTPS. Start in testing mode while you verify every active MX hostname.
Testing-mode example
version: STSv1 mode: testing max_age: 2592000 mx: mail.example.com
Testing is a careful first step. It lets compatible sending services discover your policy while you check that normal mail continues to arrive.
Enforce-mode example
version: STSv1 mode: enforce max_age: 2592000 mx: mail.example.com
Use enforce only after the MX list is complete and your policy is hosted over valid HTTPS. When you change a policy, update the DNS policy ID too.