Does MTA-STS policy affect outbound emails from my domain?

No, MTA-STS primarily applies to inbound emails. It dictates how other mail servers send email to your domain . Your domain, when acting as a sending server, retrieves the MTA-STS policies of recipient domains to ensure secure outbound delivery. It does not directly affect how your domain's outbound mail is handled unless the receiving server uses MTA-STS.

What happens if an MTA-STS policy file is unavailable or invalid?

If a sending server cannot retrieve a valid MTA-STS policy via HTTPS, it will typically fall back to opportunistic TLS or temporarily queue the message depending on the policy's mode. In enforce mode, mail delivery might fail if a secure connection cannot be established, highlighting the importance of policy accessibility.

Does MTA-STS replace DMARC, SPF, or DKIM?

No, MTA-STS works in conjunction with DMARC, SPF, and DKIM, but does not replace them. These protocols address different aspects of email security. MTA-STS ensures the transport layer security (TLS) of email, while DMARC, SPF, and DKIM focus on sender authentication and message integrity. For comprehensive protection, all should be implemented.

Does MTA-STS use SMTP or HTTPS for policy retrieval? - MTA-STS - Email authentication - Knowledge base

An illustration showing HTTPS securing email transmission by retrieving a policy file, distinct from the SMTP data flow.

Mail Transfer Agent Strict Transport Security (MTA-STS) is a crucial security standard designed to ensure that email is always sent over a secure, encrypted connection using Transport Layer Security (TLS). This helps prevent man-in-the-middle attacks and passive eavesdropping, which are common vulnerabilities in standard SMTP email delivery.

The core question of how MTA-STS retrieves its policy often leads to confusion. While MTA-STS ultimately secures SMTP traffic, the mechanism for fetching its security policy is distinct from the email transmission itself. Understanding this distinction is vital for proper implementation and ensuring robust email security for your domain.

It's important to clarify that MTA-STS leverages existing internet protocols for different stages of its operation. SMTP (Simple Mail Transfer Protocol) is the protocol for sending emails, but it's not directly involved in the policy lookup process for MTA-STS. Instead, the policy retrieval relies on web-based standards.

Policy discovery process

DNS discovery and the policy file

The process begins with DNS. When a sending mail server wants to determine if a recipient domain supports MTA-STS, it first performs a DNS query for a specific DNS TXT record. This record, usually _mta-sts, indicates whether MTA-STS is enabled for the domain and provides a unique ID for the current policy.

If the TXT record exists, the sending server then knows that the recipient domain is ready to share its MTA-STS policy. This DNS record does not contain the policy itself, but rather points the sending server to where it can find the policy. It's an initial handshake, guiding the sender to the next step of the security process.

The policy itself is a text file that outlines the rules for accepting mail. It specifies which MX (Mail Exchange) hosts are authorized to receive mail for the domain and whether those hosts must support TLS. This file is critical for enforcing the secure email delivery requirements.

How the policy is fetched

HTTPS is used for policy retrieval

The direct answer to the question is that MTA-STS uses HTTPS (Hypertext Transfer Protocol Secure) for policy retrieval. After discovering the MTA-STS DNS TXT record, the sending server attempts to fetch the policy file from a specific well-known URL on the recipient's domain. This URL follows a standard format, ensuring that all MTA-STS-compliant sending servers know exactly where to look.

Specifically, the policy file is retrieved via HTTPS from the directory path https://mta-sts.yourdomain.com/.well-known/mta-sts.txt. The use of HTTPS for this step is critical because it ensures the integrity and authenticity of the policy file itself. If an attacker could tamper with the policy file, they could undermine the entire MTA-STS protection.

MTA-STS policy file retrieval URLHTTP

GET /.well-known/mta-sts.txt HTTP/1.1
Host: mta-sts.yourdomain.com

The specific filename for an MTA-STS policy is always mta-sts.txt. This standardization is what allows different mail servers across the internet to reliably find and interpret the security policies of other domains. You can read more about the technical specifications in RFC 8461: SMTP MTA Strict Transport Security.

Why HTTPS, not SMTP, for policy

The importance of HTTPS security

Using HTTPS for policy retrieval means that the connection between the sending server and the web server hosting the MTA-STS policy file is encrypted and authenticated. This prevents attackers from modifying the policy or providing a malicious fake policy, which could trick the sending server into delivering mail insecurely or to an unauthorized destination. The integrity of this policy is paramount for MTA-STS to be effective.

Why HTTPS is vital for MTA-STS

Authenticity: Ensures the policy comes from the legitimate domain owner, preventing impersonation.
Integrity: Guarantees the policy file hasn't been tampered with during transmission.
Confidentiality: While the policy itself isn't sensitive, the connection is protected.

This reliance on HTTPS means that a domain implementing MTA-STS must have a valid TLS certificate for the mta-sts.yourdomain.com subdomain (or equivalent). The browser needs to trust this certificate for the policy retrieval to succeed. Without a properly configured HTTPS server and a valid certificate, MTA-STS cannot function correctly.

The default port for MTA-STS policy fetching is the standard HTTPS port, which is port 443. This is another example of MTA-STS integrating seamlessly with existing web infrastructure rather than inventing new protocols or ports for its policy mechanism.

SMTP vs. HTTPS in email security

How MTA-STS protects email

Once the sending server retrieves and validates the MTA-STS policy, it enforces the specified security requirements for subsequent SMTP connections to the recipient domain. This means that if the policy mandates TLS encryption, the sending server will refuse to deliver mail if the recipient's mail server does not offer a valid TLS connection.

An illustration showing a secure email connection facilitated by MTA-STS and HTTPS policy.

This policy enforcement goes beyond opportunistic TLS, which allows mail to be sent unencrypted if TLS is not available. MTA-STS ensures that secure communication is a strict requirement, significantly enhancing the security of email in transit. It's a critical component in a comprehensive email authentication strategy, working alongside DMARC, SPF, and DKIM.

Together, these standards create a layered defense against email fraud and unauthorized access. For a deeper dive into how these protocols work in concert, refer to our guide to DMARC, SPF, and DKIM. Implementing MTA-STS adds another robust layer of protection, particularly against downgrade attacks that force email into unencrypted channels. You can learn more about how Microsoft Purview uses MTA-STS.

Conclusion

Monitoring and verifying MTA-STS

Even with MTA-STS implemented, continuous monitoring is essential. Incorrectly configured DNS records, issues with the HTTPS server, or expired TLS certificates can all lead to MTA-STS failures, compromising your email security. It is crucial to have systems in place to detect and verify MTA-STS policy changes.

The challenge without proper monitoring

Without vigilant oversight, an MTA-STS implementation can become a false sense of security. Changes to DNS, web server configurations, or TLS certificate renewals can inadvertently break your policy, leaving your email vulnerable without your knowledge.

Silent failures: MTA-STS can fail silently, meaning emails might revert to insecure delivery without notification.
Manual checks: Relying on manual checks is time-consuming and prone to human error.

Solution: AI-powered DMARC monitoring

To effectively manage MTA-STS and other email authentication protocols like DMARC, a robust monitoring solution is key. Platforms like Suped offer comprehensive DMARC monitoring that helps you oversee your entire email authentication ecosystem, including MTA-STS.

Real-time alerts: Receive immediate notifications for policy changes or misconfigurations.
AI recommendations: Get actionable advice to resolve issues and strengthen your policy automatically.

By providing a unified platform for DMARC, SPF, DKIM, and blocklist monitoring, Suped streamlines the complexity of email deliverability and security. Its AI-powered recommendations and real-time alerts ensure that you are always aware of your domain's health and can react promptly to any issues affecting your MTA-STS implementation.