An MTA-STS policy is served at:

https://mta-sts.example.com/.well-known/mta-sts.txt

A policy contains fields such as version, mode, MX patterns and maximum age.

Conceptually:

version: STSv1
mode: enforce
mx: mx1.example.com
max_age: 604800

The exact syntax and matching rules matter. Validate against RFC 8461 rather than treating it like arbitrary key/value configuration.

Reference material