So, to set this up for my domain, I have to make a TXT record like this:

  _mta-sts.example.com.     TXT     "v=STSv1; id=20180928;"

Where id is just a unique identifier for the current version of the STS policy, then I configure a webserver to respond to requests for https://mta-sts.example.com/.well-known/mta-sts.txt that looks like this:

  version: STSv1
  mode: testing
  mx: mx1.example.com
  mx: mx2.example.com
  max_age: 86400

Am I understanding this right?

Also, I use postfix to send email, does anyone know how I can make postfix use these policies?