What happens if my MTA-STS certificate expires?

If your MTA-STS certificate expires, sending mail servers enforcing an MTA-STS policy will fail to validate your server's TLS connection. This will likely result in those emails being deferred or rejected, potentially causing significant email deliverability issues for your domain.

Can I use a self-signed certificate with MTA-STS?

No, you cannot use a self-signed certificate with MTA-STS. The protocol requires that the certificate chains to a publicly trusted root certificate authority. Self-signed certificates are not trusted by default by other mail servers, which would lead to validation failures.

How does MTA-STS validation differ from regular TLS validation?

MTA-STS validation builds upon regular TLS validation by adding enforcement. While standard TLS might allow connections to fall back to unencrypted or accept invalid certificates in 'opportunistic' mode, MTA-STS mandates strict validation. If the TLS certificate or connection fails any check, the mail transfer is either deferred or rejected, preventing delivery over an insecure channel.

Does MTA-STS require DNSSEC for its policy discovery?

While DNSSEC is highly recommended for overall DNS security, MTA-STS itself does not directly require DNSSEC for policy discovery . However, implementing DNSSEC can provide an additional layer of trust and integrity for your MTA-STS policy records.

Does MTA-STS require a specific root certificate authority? - MTA-STS - Email authentication - Knowledge base

Two mail servers connected by a secure, encrypted link with a padlock icon.

Mail Transfer Agent Strict Transport Security (MTA-STS) is a critical security standard designed to enforce TLS encryption for email in transit. It helps prevent downgrade attacks and Man-in-the-Middle (MITM) attacks by ensuring that sending mail servers only deliver email to receiving mail servers that offer a valid and trusted TLS certificate. A common question that arises is whether MTA-STS mandates the use of a specific root certificate authority (CA).

The short answer is no, MTA-STS does not require a specific root certificate authority. Instead, it relies on the broader concept of trust within the public key infrastructure (PKI). This means that any certificate issued by a CA that is trusted by major operating systems and web browsers will generally be acceptable. The key is that the certificate must be legitimate, unexpired, and correctly configured for the mail server's hostname.

The role of TLS certificates in MTA-STS

MTA-STS enhances the security of email communications by ensuring that a mail server presenting an MTA-STS policy will only accept incoming emails via a connection secured by TLS. This protocol addresses vulnerabilities in opportunistic TLS, where a connection might fall back to an unencrypted state if TLS negotiation fails or if the certificate presented is invalid. By publishing an MTA-STS policy, a domain declares its explicit requirement for secure email transport.

When a sending mail server attempts to deliver an email to a domain that has an MTA-STS policy, it first retrieves the policy via HTTPS. This policy specifies the expected MX (Mail Exchanger) hosts for the recipient domain and the requirement for valid TLS certificates. If the receiving server presents a certificate that does not meet these requirements, the sending server will defer or reject the email, depending on the policy mode.

Email security without MTA-STS

Opportunistic TLS: Email servers attempt TLS, but don't enforce it, allowing fallback to unencrypted connections.
Vulnerable to attacks: Susceptible to downgrade attacks where attackers force unencrypted communication.
Certificate laxity: Invalid or self-signed certificates might still be accepted, compromising security.

Email security with MTA-STS

Mandatory TLS: Email servers must use TLS, preventing unencrypted fallbacks.
Protection from attacks: Significantly reduces the risk of downgrade attacks and other interception methods.
Strict certificate validation: Requires a valid, trusted TLS certificate for connection establishment.

Trusting root certificate authorities

A TLS certificate is validated through a chain of trust. This chain leads back to a root certificate authority, which is a highly trusted entity. Operating systems and applications maintain a list of pre-installed, trusted root CAs. When a server presents a certificate, the client (in this case, the sending mail server) verifies that the certificate was issued by an intermediate CA, which in turn was certified by a trusted root CA. If this chain is intact and the root CA is on the client's trusted list, the certificate is considered valid.

For MTA-STS, the requirement is simply that the destination mail server's certificate must chain to a trusted root certificate authority. This is not a new requirement introduced by MTA-STS, but rather a fundamental aspect of how TLS works. It ensures that the certificate itself is not self-signed or issued by an untrusted source, which could be exploited by attackers.

Example of a certificate chain (simplified)text

End-entity certificate (your mail server)
  -> Intermediate CA certificate
    -> Root CA certificate (trusted by OS/browser)

Best practice for certificate management

Always obtain your TLS certificates from a reputable, publicly trusted Certificate Authority. Ensure they are kept up-to-date and renewed before expiration. Regularly review your certificate configuration to prevent outages or security warnings.

Key certificate requirements for MTA-STS

While MTA-STS doesn't demand a specific root CA, it does impose other crucial requirements on the TLS certificate itself. Adhering to these requirements is vital for a successful MTA-STS implementation and to avoid email delivery issues. A foundational RFC, RFC 8461, details these specifications.

Valid and unexpired: The certificate must be currently valid and not expired. An expired certificate will cause validation failures.
Common name or SAN match: The Common Name (CN) or a Subject Alternative Name (SAN) in the certificate must match the hostname of the receiving mail server as specified in your MTA-STS policy.
Trusted chain: As discussed, the certificate must chain up to a root CA that is trusted by the sending mail server's operating system or trust store.

It is crucial to understand that even if MTA-STS works with any TLS certificate from a trusted authority, adherence to these specific conditions ensures the robust security MTA-STS aims to provide. Failing to meet any of these will result in MTA-STS validation failures, potentially affecting your email deliverability.

A digital certificate with a golden seal, representing trust.

Scenario	MTA-STS behavior	Outcome
Valid certificate from trusted CA	Passes validation	Email delivered securely
Expired certificate	Fails validation	Email deferred or rejected
Common name mismatch	Fails validation	Email deferred or rejected
Self-signed or untrusted CA certificate	Fails validation	Email deferred or rejected

Monitoring MTA-STS and certificate health

Even with a correctly configured MTA-STS policy and a valid TLS certificate, ongoing monitoring is essential. Certificates expire, configurations can change, and issues can arise that might disrupt your secure email flow. Without proper vigilance, you might experience email deliverability problems, where emails are unexpectedly blocked or diverted due to failed MTA-STS validation. This is particularly important for preventing your emails from going to spam.

Tools like Suped provide comprehensive DMARC monitoring that can help you detect MTA-STS related issues. By analyzing your DMARC reports, you can identify if sending domains are encountering problems validating your MTA-STS policy, including certificate issues. These insights are crucial for maintaining strong email deliverability rates and protecting your brand reputation.

Suped offers AI-powered recommendations that go beyond mere data presentation. The platform provides actionable steps to fix issues, including those related to MTA-STS policy misconfigurations or certificate problems. With real-time alerts and a unified dashboard for DMARC, SPF, and DKIM, it simplifies the complex task of email authentication and deliverability, making it accessible for businesses of all sizes, and a powerful solution for MSPs managing multiple clients.

Ensuring secure email delivery

In summary, MTA-STS does not require a specific root certificate authority, but it absolutely relies on a certificate issued by a globally trusted CA. The integrity of the certificate chain, its validity period, and the correct matching of its common name or Subject Alternative Name to your mail server's hostname are all critical for successful MTA-STS implementation. These factors ensure that your emails are protected against tampering and unauthorized interception during transit.

Implementing MTA-STS is a vital step in bolstering your email security posture, working alongside other protocols like DMARC, SPF, and DKIM. Regular monitoring and proactive management of your TLS certificates are crucial to maintaining a secure and reliable email infrastructure. By understanding and adhering to these requirements, you can significantly enhance the security and trustworthiness of your email communications.