You need to have a correctly formatted DNS TXT record at _mta-sts.example.com first. They'll check for that, and only if it exists will they bother attempting t

You need to have a correctly formatted DNS TXT record at _mta-sts.example.com first. They'll check for that, and only if it exists will they bother attempting to fetch your mta-sts policy from https://mta-sts.example.com/.well-known/mta-sts.txt https://mta-sts.example.com/.well-known/mta-sts.txt