SMTP encryption is opportunistic by default, which means it is optional, which means it is strippable. A sending server connects, looks for STARTTLS in the EHLO response, and upgrades if it sees it. Remove that one line in transit and the sender falls back to plaintext without complaint, because it has no way to know TLS was ever supposed to happen. MTA-STS is how you tell it.

Why STARTTLS Alone Is Not Enough

The gap is that opportunistic TLS has no expectation to violate. A sender that fails to negotiate TLS has no basis to refuse delivery, since plenty of legitimate mail servers still do not offer it. Certificate validation is usually skipped for the same reason: a mail server presenting a self-signed or expired certificate is common enough that treating it as fatal would break real delivery.

So the default posture is an encrypted channel to an unverified party, downgradeable by anyone in the path. MTA-STS, specified in RFC 8461, lets a receiving domain publish a policy saying TLS is required, the certificate must validate, and here are the hostnames allowed to receive mail. Senders that implement it will refuse to deliver rather than fall back.

The Three Pieces

First, a DNS TXT record at the _mta-sts label telling senders a policy exists:

_mta-sts.example.com.  IN  TXT  "v=STSv1; id=20260916120000Z;"

The id is the whole mechanism for cache invalidation. Senders compare it against the one they saw last time, and they only re-fetch the policy when it changes. Bump it on every policy edit or your change will not be noticed until caches expire on their own.

Second, the policy itself, served over HTTPS from a specific host and path:

https://mta-sts.example.com/.well-known/mta-sts.txt

version: STSv1
mode: enforce
mx: mail.example.com
mx: mail2.example.com
max_age: 604800

mode takes enforce, testing, or none. Every MX that can receive mail for the domain needs a line, and a wildcard like *.example.com matches only the leftmost label, so it covers mail.example.com and not foo.bar.example.com. max_age is in seconds with a ceiling of 31557600, about a year.

Third, and this is the part people trip over: the mta-sts host serving that file needs a valid, publicly trusted certificate of its own. The policy is only as trustworthy as the HTTPS connection that delivered it, so a sender that cannot validate that certificate discards the policy entirely. You have now made your mail delivery depend on a certificate on a host that has nothing else to do with mail, which is exactly the kind of endpoint that quietly expires.

Start in Testing Mode

Publishing mode: enforce as the first step is how you find out about your forgotten backup MX by having mail to it rejected. mode: testing makes senders evaluate the policy, report failures, and deliver anyway.

Leave it there long enough to see a full cycle of your real traffic. A week is reasonable, longer if you have partners who send in bursts. The reports are the point of the exercise, which brings up the other half.

TLS-RPT Tells You What Broke

RFC 8460 defines a companion record that asks senders to report what happened. It is cheap to add and it is the only feedback channel you get:

_smtp._tls.example.com.  IN  TXT  "v=TLSRPTv1; rua=mailto:tlsrpt@example.com"

Reports arrive as JSON, daily, from each sending organization that implements it. The useful part is the failure detail:

{
  "organization-name": "Example Sender Inc",
  "date-range": { "start-datetime": "2026-09-16T00:00:00Z",
                  "end-datetime":   "2026-09-16T23:59:59Z" },
  "report-id": "2026-09-16T00:00:00Z_example.com",
  "policies": [{
    "policy": { "policy-type": "sts", "policy-domain": "example.com" },
    "summary": { "total-successful-session-count": 8214,
                 "total-failure-session-count": 17 },
    "failure-details": [{
      "result-type": "certificate-host-mismatch",
      "sending-mta-ip": "203.0.113.9",
      "receiving-mx-hostname": "mail2.example.com",
      "failed-session-count": 17
    }]
  }]
}

The result-type values are specific enough to act on directly. starttls-not-supported, certificate-expired, certificate-host-mismatch, certificate-not-trusted, validation-failure, and on the policy side sts-policy-fetch-error, sts-policy-invalid, and sts-webpki-invalid. That last group points at your policy host rather than your mail servers, which is a distinction worth internalizing before you start debugging the wrong machine.

The Ways This Bites

Caching cuts both ways. Once a sender has your policy with a week-long max_age, a mistake persists for that long even after you fix the file, unless the sender happens to re-check the id. Adding an MX means updating the policy and bumping the id before the new host starts receiving, and doing it in that order.

The certificate on the mta-sts web host is the failure nobody plans for. It is usually a static file on a server that gets less attention than the mail infrastructure, and when it expires, compliant senders stop trusting your policy. Whether that means mail is deferred or the policy is simply ignored depends on the sender.

Your MX certificates now genuinely have to be correct as well. Names have to match what the policy lists, chains have to be complete, and expiry stops being cosmetic. That is the point of turning it on, and it is worth knowing before you flip to enforce.

To check what you have published, the MTA-STS checker on mrdns.com fetches the record and the policy together and tells you whether they agree. For the mail server configuration underneath it, goodtls.com has per-application guides covering Postfix, Exim, Dovecot, and the rest.