What are the components of MTA-STS (policy file, DNS record)?
MTA STS has two main components:
A DNS TXT record at the dedicated \_mta-sts.yourdomain.com subdomain for your domain. This record announces that an MTA STS policy exists and specifies the policy version and mode.
A policy file served over HTTPS at a specific well known path: https://mta-sts.yourdomain.com/.well-known/mta-sts.txt.
The DNS record is the public notice. The HTTPS file contains the actual rules.
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