What happens if my MTA-STS policy is in 'enforce' mode and an email fails a security check?

If an email fails a security check (e.g., no TLS, invalid certificate) when your MTA-STS policy is in 'enforce' mode, the sending mail server will refuse to deliver the email. It will either queue the message for a later retry attempt or issue a non-delivery report (NDR) to the original sender. This ensures that only securely transmitted emails reach your domain.

How long should I run in 'testing' mode before switching to 'enforce'?

The duration of your 'testing' phase depends on your email volume and complexity. A common recommendation is to run in 'testing' mode for at least a few weeks, or even months, to ensure you capture data from all sending partners and identify any potential issues without impacting email delivery. This allows you to address problems before switching to the strict 'enforce' mode.

Does 'enforce' mode protect against email spoofing?

'Enforce' mode primarily secures the transport layer of email, protecting against man-in-the-middle and downgrade attacks. While it enhances overall email security, it does not directly prevent email spoofing (where a sender impersonates your domain). For spoofing protection, you need to implement DMARC , SPF, and DKIM.

What tools can help monitor MTA-STS 'enforce' mode compliance?

DMARC monitoring platforms like Suped are essential for monitoring MTA-STS compliance. They process aggregated DMARC reports, which contain data on MTA-STS failures. These platforms provide dashboards and alerts that help you quickly identify and troubleshoot any issues preventing secure email delivery to your domain.

What is the 'enforce' mode in MTA-STS? - MTA-STS - Email authentication - Knowledge base

Secure email connection secured by MTA-STS enforce mode

Email is a cornerstone of modern communication, but its security has historically been vulnerable to attacks like man-in-the-middle (MITM) and downgrade attacks. Mail Transfer Agent Strict Transport Security (MTA-STS) was introduced to combat these threats by enforcing secure connections for email delivery. At its core, MTA-STS relies on a policy file published by a domain owner, which dictates how sending mail servers should interact with their inbound mail infrastructure.

The operational behavior of an MTA-STS policy is defined by its 'mode' field. This field is crucial because it tells external mail servers how strictly they should adhere to the policy's rules regarding secure connections. While there are three possible modes, 'enforce' mode is the ultimate goal for most organizations aiming for maximum email transport security. Understanding the nuances of this mode is vital for successful implementation and robust email protection.

When deploying MTA-STS, you typically progress through different modes to ensure a smooth transition and avoid mail flow disruptions. This phased approach helps validate your configuration before you fully commit to the most stringent settings. For a comprehensive overview of how this critical field operates, it helps to understand what the 'mode' field in an MTA-STS policy means.

The three MTA-STS policy modes

Before diving into 'enforce' mode, it's helpful to briefly consider the three possible 'mode' values in an MTA-STS policy. Each serves a distinct purpose in the deployment lifecycle of MTA-STS. The initial 'none' mode indicates that MTA-STS is not active, and recipients can deliver email without requiring TLS. This is generally used when an organization does not want to implement MTA-STS or wishes to temporarily disable it.

The 'testing' mode allows senders to check for MTA-STS compliance without rejecting messages that fail verification. It's an essential intermediate step, letting you gather valuable feedback on your policy's effectiveness and identify potential issues without impacting email delivery. When an email fails during 'testing' mode, it's still delivered, but the sender records the failure, typically via DMARC reports, which are crucial for monitoring. To learn more about this interim step, explore what the 'testing' mode in MTA-STS is.

Finally, 'enforce' mode is the most secure setting. It dictates that all incoming mail must be delivered over a secure TLS connection, and the server's certificate must be valid and trusted. If these conditions are not met, the sending mail server will not deliver the email. This strictness is what provides significant protection against various email-borne threats. Understanding what the 'none' mode in MTA-STS means can also clarify the contrast with 'enforce' mode.

What 'enforce' mode means for email delivery

When your MTA-STS policy is set to 'enforce', you are explicitly instructing other mail servers to prioritize security when sending email to your domain. This means that if a sending server attempts to establish a connection that doesn't meet the specified security requirements (e.g., no TLS, untrusted certificate), the email delivery will fail. The sending server will typically queue the message for later retry or return a non-delivery report (NDR) to the original sender, depending on its configuration.

This strict enforcement is critical for protecting against sophisticated attacks. For instance, 'enforce' mode helps protect against downgrade attacks, where an attacker tries to force a less secure or unencrypted connection, and man-in-the-middle attacks, where an attacker attempts to intercept and potentially alter emails in transit. By rejecting insecure connections outright, 'enforce' mode ensures that only trusted and encrypted channels are used, enhancing the integrity and confidentiality of your inbound email traffic. It effectively protects against downgrade attacks.

The benefits of 'enforce' mode extend beyond just preventing direct attacks. It also signals to the broader email ecosystem that your domain takes email security seriously. This can indirectly improve your domain's reputation and trust among other mail service providers. As email security standards evolve, adopting 'enforce' mode becomes an increasingly important part of a robust email authentication strategy, complementing other protocols like DMARC, SPF, and DKIM.

Testing mode

No email disruption: Emails are still delivered even if TLS or certificate validation fails.
Monitoring required: Requires active monitoring of DMARC reports for MTA-STS failure data.
Risk of compromise: Vulnerable to downgrade attacks as insecure connections are allowed.
Learning phase: Ideal for initial deployment to identify and fix configuration issues.

Enforce mode

Strict enforcement: Emails failing TLS or certificate checks are rejected, not delivered.
Enhanced security: Provides strong protection against MITM and downgrade attacks.
Potential for mail loss: Misconfigurations can lead to legitimate emails being rejected.
High assurance: Ensures all incoming mail transport is encrypted and authenticated.

Implementing 'enforce' mode

Transitioning to 'enforce' mode requires careful planning and execution. The process typically involves three key steps: publishing an MTA-STS DNS TXT record, creating and hosting an MTA-STS policy file, and then monitoring the results. The DNS TXT record announces your domain's support for MTA-STS and specifies the version and ID of your policy. The policy file itself, which must be hosted on a well-known HTTPS endpoint, contains the actual rules, including the 'mode' parameter, like policy file names.

Example MTA-STS policy fileYAML

version: STSv1
mode: enforce
mx:
  - mx1.example.com
  - mx2.example.com
max_age: 86400

Once your policy is published with 'mode: enforce', sending servers that support MTA-STS will fetch your policy and abide by its rules. If your MX records or certificate chain are not configured correctly, legitimate email could be deferred or rejected. It's crucial to test thoroughly in 'testing' mode first and ensure your mail infrastructure is fully compliant before making the switch to 'enforce'. We see many customers rush this step and run into deliverability issues.

Key considerations for 'enforce' mode

Valid TLS certificates: Ensure all mail servers listed in your policy use valid, publicly trusted TLS certificates.
MX record accuracy: The 'mx' field in your policy must accurately list all mail exchange hosts for your domain.
Policy file accessibility: Your policy file must be accessible via HTTPS on the well-known URL provided in your DNS TXT record.
Thorough testing: Always run in 'testing' mode for an extended period to catch all issues.

Monitoring and impact on deliverability

Even after successfully deploying 'enforce' mode, continuous monitoring is non-negotiable. MTA-STS failures, while preventing insecure delivery, can lead to legitimate emails being bounced or delayed. You need a way to track these failures and respond quickly to any changes in your infrastructure or policy. This is where DMARC reports, specifically those with MTA-STS failure data, become invaluable. These reports provide insight into which sending domains are encountering issues when trying to connect securely to your servers.

Email server with MTA-STS protection ensuring secure email delivery

A robust DMARC monitoring solution is essential for managing MTA-STS. Suped offers a comprehensive DMARC monitoring platform with AI-powered recommendations that can help you detect and resolve MTA-STS issues quickly. Our platform unifies DMARC, SPF, and DKIM monitoring, providing real-time alerts and actionable insights to ensure your email deliverability remains high, even with stringent 'enforce' policies. Our generous free plan allows you to start protecting your domain without any upfront cost.

Field	Description	Required
version	Specifies the version of the MTA-STS standard being used. Currently STSv1.	Yes
mode	Sets the policy to 'none', 'testing', or 'enforce' mode.	Yes
mx	A list of MX hostnames that receive email for the domain, requiring TLS.	Yes
max_age	The number of seconds a policy should be cached by sending servers.	Yes

Embracing secure email delivery

'Enforce' mode in MTA-STS is a powerful mechanism for securing your inbound email traffic. By requiring all connections to use TLS and valid certificates, it significantly reduces the risk of email interception, tampering, and other forms of cyber attack. While the implementation requires precision and thorough testing, the security benefits far outweigh the complexities.

Adopting MTA-STS 'enforce' mode, in conjunction with other email authentication protocols like DMARC, SPF, and DKIM, provides a layered defense that protects your domain and your recipients. This commitment to secure email transport is becoming a standard expectation for trustworthy organizations in today's digital landscape. Microsoft has even provided guidance on enhancing mail flow with MTA-STS.