What Are MTA-STS and TLS Reporting, and Do I Need Them?

MTA-STS and TLS reporting are protocols that enforce encrypted connections between mail servers and report when encryption fails. They protect email in transit, not sender identity. Most MSPs and MSSPs should implement SPF, DKIM, and DMARC first, then add MTA-STS and TLS-RPT as a later hardening layer once authentication is solid.

What problem does MTA-STS solve?

STARTTLS is opportunistic by default, meaning attackers may attempt downgrade attacks that force delivery over an unencrypted connection (easydmarc.com). MTA-STS tells compliant sending mail servers to deliver email to your domain only over TLS when a valid policy is in place. Unlike SPF, DKIM, and DMARC, which verify sender identity, MTA-STS and TLS-RPT protect the encryption of emails already in transit between mail servers (fluentsmtp.com).

SPF, DKIM, and DMARC stop someone from forging your address. MTA-STS protects the message while it moves (dmarcdkim.com). If a client sends you a purchase order or contract, DMARC confirms it came from their domain. MTA-STS ensures nobody read or modified it while it crossed the internet. Across the top one million domains, MTA-STS adoption is still below 1 percent, making this an area where most organizations lag.

How does MTA-STS work?

A working MTA-STS deployment is made of three pieces: a DNS TXT record that signals the policy exists, a policy file hosted over HTTPS that carries the actual rules, and a TLS-RPT record that collects failure reports (bouncecheck.email).

The DNS record lives at _mta-sts.yourdomain.com and contains only a version string and an ID you increment whenever you change the policy. The policy file is hosted at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt, requires a valid TLS certificate, and lists which MX servers are authorized, the policy mode, and how long to cache the policy.

The policy mode can be testing, enforce, or none. In testing mode, senders send reports indicating policy application failures, but TLS connection failures will not be blocked (dmarcian.com). In enforce mode, failing TLS means that emails will not be delivered. If an MX is missing from the policy or there is a certificate issue, your inbound email starts being silently rejected, so the right way is to start with mode testing, watch the TLS-RPT reports, and move to enforce only once it is clean (kinetikbilisim.net).

What does TLS-RPT tell you?

TLS-RPT is a reporting standard, defined in RFC 8460, that lets domain owners receive regular, structured reports on TLS connection failures for emails sent to their domain, telling you when they happened, how often, and why, so you can fix the underlying problem (powerdmarc.com). The record is a TXT entry at _smtp._tls.yourdomain.com pointing to an email address where you want reports delivered.

A TLS-RPT report is a once-per-UTC-day JSON aggregate that a sending MTA emails to a recipient domain, summarising how many SMTP sessions opened or failed to open TLS against that domain's MX hosts and records which MTA-STS or DANE policy was applied to each one. Reports arrive as gzipped JSON files showing which sending server attempted delivery, which of your MX hosts it connected to, what type of failure occurred (expired certificate, hostname mismatch, or TLS version incompatibility), and a count of affected messages.

TLS reports highlight issues like expired certificates, STARTTLS failures, and policy mismatches, making troubleshooting faster and more accurate.

Do you need both?

Technically no, but practically yes. MTA-STS without TLS-RPT means you are protected but blind. Always deploy them together. Without TLS-RPT, you will not know if a legitimate sender is failing to reach you because of a certificate problem, a misconfigured MX record, or an incomplete policy file. The reports are what make MTA-STS operational rather than theoretical.

Most vendors offer hosted MTA-STS services that handle the HTTPS endpoint and parsing of TLS-RPT JSON for you. If you manage your own, you need web hosting with a valid certificate for the mta-sts subdomain, a way to serve a static text file at the required path, and something to receive and parse daily compressed JSON reports.

When should an MSP or MSSP deploy this?

After SPF, DKIM, and DMARC are deployed and enforced. A mature inbound posture often looks like SPF, DKIM, and DMARC protecting domain identity and visible-author trust, MTA-STS publishing strict expectations for inbound SMTP TLS, and TLS-RPT providing visibility when those expectations fail in the wild. If a client does not yet have a DMARC policy at p=reject, fixing that closes a larger attack surface than MTA-STS does.

Prioritize MTA-STS for clients in regulated sectors where message confidentiality carries compliance weight (finance, healthcare, legal, government), clients who exchange commercially sensitive documents by email, and any organization whose email authentication stack is already solid. If the free AI-visibility check (/check) shows that competitor visibility or technical gaps are the bigger issue, solve those before spending time on transit encryption.

For white-label reporting, ActiScore's pricing (/pricing) includes DNS and TLS certificate health on every plan. Hosting the MTA-STS policy file and parsing TLS-RPT reports still requires separate infrastructure or a third-party service. If you are building a managed security offering around email, MTA-STS and TLS-RPT are worth the setup cost once everything upstream is already locked down. If not, focus on the guide (/guide) to authentication first and revisit this as a later hardening step.