CloudFlare Worker for MTA-STS policy

mta-sts.js

const mode= 'testing';
//const mode= 'enforce';
const max_age= 604800; // 1 week
const mx_list = [
   'aspmx.l.google.com'
  ,'aspmx2.googlemail.com'
  ,'aspmx3.googlemail.com'
  ,'aspmx4.googlemail.com'
  ,'aspmx5.googlemail.com'
  ,'alt1.aspmx.l.google.com'
  ,'alt2.aspmx.l.google.com'
];

const sts= `version: STSv1
mode: ${mode}
${mx_list.map(i=> 'mx: '+i).join('\n')}
max_age: ${max_age}`;

addEventListener('fetch', evt=> {
  return evt.respondWith(new Response(sts));
});

Don't forget to add the route to this worker: https://[example.com]/.well-known/mta-sts.txt

Thanks a lot @dotysan!

I had to adjust the mx_list slightly:

const mode = 'testing';
// const mode = 'enforce';
const max_age = 604800; // 1 week
const mx_list = [
   'aspmx.l.google.com',
   'alt1.aspmx.l.google.com',
   'alt2.aspmx.l.google.com',
   'alt3.aspmx.l.google.com',
   'alt4.aspmx.l.google.com'
];

const sts = `version: STSv1
mode: ${mode}
${mx_list.map(i=> 'mx: '+i).join('\n')}
max_age: ${max_age}`;

addEventListener('fetch', evt=> {
  return evt.respondWith(new Response(sts));
});

And add an A record to be able to route the traffic to this worker:

• Type: A
• Name: mta-sts
• IPv4 address: 192.0.2.1
• Proxy status: On (this is important!)
• TTL: Auto