HUGE THANKS TO JIMEH FOR THE TEMPLATE: [https://github.com/jimeh/mta-sts-on-github-pages/tree/main]
This serves as alternative supplier option in step 3 described here: [https://www.security.gov.uk/policy-and-guidance/set-up-tls-rpt-and-mta-sts/]
CNAME DNS record at mta-sts.YOURDOMAIN pointing to GITHUB_USER.github.io..TXT DNS record at _mta-sts.YOURDOMAIN indicating the use of MTA-STS, and update the id value on policy change.YOURDOMAIN with your custom domain in CNAME..well-known/mta-sts.txt to set mx directives to match the MX DNS records of your domain./) of the main branch as source.