Email & DNS
MTA-STS setup: publish and test an HTTPS mail transport policy
MTA-STS lets a domain publish a policy telling supporting senders to require TLS and expect specific MX hostnames after the policy has been fetched over HTTPS. It strengthens SMTP transport policy, but it depends on DNS, HTTPS hosting, valid certificates, and mail routing all remaining coordinated.
Start with stable MX and TLS
Before enforcing MTA-STS, verify the MX hosts are intentional, publicly reachable on SMTP, and present certificates valid for the names the policy will authorize. Enforcement on top of broken TLS can delay legitimate mail.
Inventory backup MX hosts too; they must be represented if senders may use them.
Publish the policy at the required HTTPS hostname
The policy is served from the standardized mta-sts hostname and path over HTTPS. It includes version, mode, allowed mx patterns, and max_age. The web certificate for the policy host must itself validate normally.
Use testing mode first so policy problems can be observed without immediate enforcement behavior.
Use the DNS TXT record to signal policy changes
The _mta-sts TXT record carries a policy id that senders can use to notice updates. Change the id when the policy meaning changes so cached senders have a reason to refetch.
DNSSEC is helpful for DNS integrity but MTA-STS is designed around its HTTPS policy fetch and certificate validation model.
Monitor before moving to enforce
Combine TLS-RPT reports, SMTP logs, and external tests to identify mismatched MX names or TLS failures. Only move to enforce when legitimate inbound routing consistently satisfies the policy.
- Stabilize MX/TLS first.
- Serve the policy over valid HTTPS.
- Increment policy id on changes.
- Test before enforce.