What kind of TLS certificate is required for MTA-STS?

MTA-STS requires a publicly trusted TLS certificate issued by a recognized Certificate Authority. Self-signed certificates are not compatible.

Why can't I use a self-signed certificate with MTA-STS?

Self-signed certificates lack external validation, meaning there's no way for a sending server to independently verify the receiving server's identity, which is a core security function of MTA-STS.

What happens if my TLS certificate expires?

An expired TLS certificate will cause MTA-STS validation to fail, leading to email delivery rejections or deferrals when your policy is in enforce mode .

How can I monitor my MTA-STS certificate compliance?

Regularly check your DMARC reports for TLS-related failures and use tools like Suped for comprehensive email authentication monitoring and real-time alerts.

Does MTA-STS work with any TLS certificate? - MTA-STS - Email authentication - Knowledge base

Q: Does MTA-STS validate the MX records?

Yes, MTA-STS validates that the MX record hostname specified in the policy file matches the hostname in the presented TLS certificate. It's important to know that MTA-STS also validates the MX records for a domain as part of its process.

An envelope secured with a padlock, symbolizing MTA-STS email security.

When implementing MTA-STS (Mail Transfer Agent Strict Transport Security), a common question arises: can I use any TLS certificate? The short answer is no. MTA-STS is designed to prevent downgrade attacks and ensure that email is always transmitted over encrypted connections using valid, trusted certificates. This means there are specific requirements that your TLS certificate must meet for MTA-STS to function correctly and provide the intended security benefits.

MTA-STS works by enabling sending email servers to verify that the receiving server supports TLS and presents a valid certificate. This validation process is crucial for preventing attackers from intercepting or tampering with emails by forcing unencrypted connections or presenting forged certificates. Without the right certificate, MTA-STS policies will fail, potentially causing delivery issues and undermining your email security posture.

Understanding these certificate requirements is fundamental to a successful MTA-STS deployment. It directly impacts your email deliverability and helps safeguard your communications against various threats. Let's delve into the specifics of what makes a TLS certificate compatible with MTA-STS.

Certificate requirements for MTA-STS

The role of TLS certificates in MTA-STS

For MTA-STS to be effective, the receiving mail server must present a TLS certificate that is publicly trusted. This isn't just a suggestion, it's a core requirement. A publicly trusted certificate is one that has been issued by a Certificate Authority (CA) that is recognized and trusted by major operating systems and web browsers. This trust is established through a chain of trust back to a root CA. Any email server attempting to send mail to your domain will validate this certificate against its list of trusted root CAs.

If a server presents a certificate that is not publicly trusted (for instance, a self-signed certificate), the sending MTA (Mail Transfer Agent) will reject the connection, treating it as a potential man-in-the-middle attack. This protective measure is precisely why MTA-STS is so powerful, as it eliminates opportunistic TLS and enforces a higher standard of security. Microsoft provides documentation on enhancing mail flow with MTA-STS which emphasizes these security aspects.

Furthermore, the certificate must be valid for the hostname of the MX record(s) specified in your MTA-STS policy file. This means the Common Name (CN) or a Subject Alternative Name (SAN) field within the certificate must match the MX hostname exactly. If there's a mismatch, the validation will fail. This requirement ensures that the certificate presented belongs to the intended mail server, preventing attackers from using certificates issued for other domains.

Key certificate attributes for MTA-STS

Publicly trusted: Issued by a recognized Certificate Authority (CA).
Valid hostname: Matches the MX record hostname in the policy file (CN or SAN). This is critical for preventing security failures, and you can learn more about MTA-STS and its certificate authority requirements for more details.
Not expired: Must be within its validity period. An expired certificate is treated as invalid.
Correct key usage: Should be suitable for server authentication.

Publicly trusted certificates are essential

Publicly trusted certificates vs. self-signed certificates

The distinction between publicly trusted and self-signed certificates is fundamental to MTA-STS's operation. While self-signed certificates can provide encryption, they lack the third-party validation that a trusted CA offers. This lack of external validation makes them unsuitable for MTA-STS, as there's no reliable way for a sending server to confirm the identity of the receiving server.

Publicly trusted certificates

Validation: Issued by a globally recognized Certificate Authority.
Trust: Automatically trusted by most email clients and servers.
MTA-STS compatibility: Required for successful MTA-STS implementation.

Self-signed certificates

Validation: Signed by the server itself, no third-party validation.
Trust: Not automatically trusted, often flagged as insecure.
MTA-STS compatibility: Incompatible with MTA-STS and will cause failures.

Some technologies, like DANE, can work with self-signed certificates in specific configurations, but MTA-STS explicitly requires publicly trusted ones. This design choice simplifies the trust model, making it easier for MTA-STS-enabled sending servers to verify the authenticity of the receiving server's certificate. Google also highlights the need for valid public certificates for MTA-STS.

Therefore, if you are planning to implement MTA-STS, ensuring all your mail servers present valid, unexpired, and publicly trusted TLS certificates matching their respective MX hostnames is a non-negotiable step. This is a critical component for protecting against eavesdropping and ensuring your emails reach their intended recipients securely, free from potential blocklist (or blacklist) issues.

MTA-STS policy and validation

The MTA-STS policy file, retrieved via HTTPS, plays a central role in communicating your domain's TLS requirements. This file explicitly lists the MX hostnames that should be protected and their expected certificates. The format of the MTA-STS policy file is a crucial detail for ensuring proper configuration. When a sending server initiates an email transfer, it first queries your domain's MTA-STS TXT record, then fetches this policy file. The policy includes a mode (enforce, testing, or none) and a list of your designated MX hosts along with their certificate fingerprints (or hashes, though less common now).

Example MTA-STS policy fileYAML

version: TLSRPTv1
mode: enforce
mx:
  - hostname: mail.yourdomain.com
    service: smtp
    port: 25
    cert_hashes:
      - sha256: 62d1c6c0b3d875a02e861d9a2d8a5996b79c3d4e7d4d4e0e8c0c1b4b1a4a1a4a
  - hostname: backup-mail.yourdomain.com
    service: smtp
    port: 25
    cert_hashes:
      - sha256: f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7f8d7
max_age: 604800

The MTA-STS policy file's name is fixed as mta-sts.txt, and it must be served from a specific directory path. Upon fetching the policy, the sending MTA then attempts to establish a TLS connection to the listed MX hosts. During the TLS handshake, it rigorously checks the presented certificate against the policy's requirements. This includes verifying that the certificate is publicly trusted, not expired, and that its hostname matches the MX entry. The certificate chain is also validated back to a trusted root CA.

If any of these validation steps fail, the sending server will refuse to deliver the email, instead following the instructions specified by the policy's mode. In enforce mode, this typically means a hard bounce or deferral. This robust validation process is what allows MTA-STS to effectively protect against downgrade attacks, ensuring that email is only exchanged over secure, authenticated connections.

Maintaining compliance and deliverability

Ensuring MTA-STS compliance and deliverability

Maintaining MTA-STS compliance requires diligent monitoring of your TLS certificates. Expired or improperly configured certificates are a leading cause of MTA-STS failures. It's not enough to simply deploy MTA-STS, you must also have a process in place to track certificate expiry dates and ensure renewals are handled promptly. This is part of a broader strategy to boost email deliverability rates.

A magnifying glass inspecting a secure network connection, symbolizing MTA-STS monitoring.

Furthermore, MTA-STS works by applying to inbound mail, so its success heavily relies on how other MTAs perceive your server. Using a DMARC reporting tool like Suped can provide valuable insights into certificate-related issues. While DMARC primarily focuses on sender authentication, it also tracks TLS failures, which can indirectly indicate problems with your MTA-STS configuration or certificates. Suped's unified platform offers AI-powered recommendations to resolve issues quickly, helping you maintain optimal email security and deliverability.

Regularly reviewing your DMARC reports (even though MTA-STS and DMARC are distinct protocols) alongside dedicated MTA-STS monitoring can help identify and rectify certificate problems before they lead to significant email delivery disruptions or security vulnerabilities. Suped also offers DMARC monitoring and SPF flattening, along with real-time alerts for comprehensive email security management.

Key takeaways

Conclusion

In summary, MTA-STS does not work with just any TLS certificate. It explicitly requires a publicly trusted, valid, and unexpired TLS certificate that matches the MX hostnames specified in your MTA-STS policy. This stringent requirement is what gives MTA-STS its strength in preventing man-in-the-middle attacks and ensuring secure, encrypted email delivery. Self-signed certificates, while useful for other purposes, are incompatible with MTA-STS's trust model.

For organizations leveraging MTA-STS, continuous monitoring of certificate validity and policy compliance is vital. Tools like Suped can help you gain visibility into your email authentication protocols and promptly address any issues, ensuring your email ecosystem remains secure and your messages consistently reach the inbox.