What happens if I set my MTA-STS policy to 'enforce' without prior testing?

Setting your MTA-STS policy directly to enforce without prior testing in testing mode can lead to significant email delivery failures. If there are any misconfigurations in your policy or issues with your mail servers' TLS setup, compliant sending servers will defer or reject email, resulting in lost or delayed messages. It's highly recommended to use the testing mode first to identify and resolve issues before full enforcement.

How often should I review my TLS-RPT reports when my policy is in 'testing' mode?

You should review your TLS-RPT reports frequently, ideally daily, especially during the initial deployment phase of testing mode. This allows you to quickly detect any issues with your MTA-STS policy or TLS configuration that might prevent secure email delivery. Consistent monitoring helps ensure a smooth transition to enforce mode.

Can I switch directly from 'enforce' mode back to 'none' if I encounter problems?

Yes, you can switch from enforce mode to none if you encounter severe issues. This will effectively disable MTA-STS enforcement, allowing email to flow without strict TLS requirements. However, be aware that there is a caching period (determined by your max_age value) during which some sending servers might still attempt to enforce the old policy. It is better to transition to testing first to diagnose problems.

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

No, MTA-STS does not replace DMARC, SPF, or DKIM. Instead, it complements these email authentication protocols. DMARC, SPF, and DKIM primarily focus on sender authentication and domain reputation, while MTA-STS ensures the secure transport of email between servers. All of these protocols work together to provide comprehensive email security and prevent phishing and spoofing attacks.

What are the three possible 'mode' values in an MTA-STS policy? - MTA-STS - Email authentication - Knowledge base

Stylized envelope with a shield, representing MTA-STS email security.

MTA-STS, or Mail Transfer Agent Strict Transport Security, is a crucial standard designed to enhance the security of email communications. It ensures that mail is sent over a secure, encrypted connection, protecting it from various attacks like man-in-the-middle and passive eavesdropping. At the heart of MTA-STS is its policy file, which dictates how sending mail servers should interact with your domain.

The policy defines several parameters, including the Mail Exchanger (MX) records and, importantly, the policy mode. This mode specifies how rigorously an MTA-STS compliant sending server should apply your stated security policy when delivering email to your domain. Understanding these modes is fundamental to correctly deploying and managing MTA-STS.

There are three distinct mode values available within an MTA-STS policy: none, testing, and enforce. Each serves a specific purpose in the lifecycle of your MTA-STS deployment, from initial setup and monitoring to full enforcement. Getting the mode field in an MTA-STS policy right is key to preventing email delivery issues while maximizing security.

The 'none' mode

The 'none' mode: disabling MTA-STS

The none mode in MTA-STS effectively disables MTA-STS for your domain. When a sending mail server fetches a policy with the mode set to none, it understands that you do not wish to enforce secure transport via MTA-STS. This means it will revert to standard SMTP behavior, which may or may not use opportunistic TLS, without strict enforcement.

While it might seem counterintuitive to disable a security feature, the none mode is vital for specific scenarios. It's often used when you need to temporarily pause MTA-STS enforcement, perhaps due to maintenance, or if you've discovered an issue with your policy that is inadvertently blocking legitimate email. Setting the mode to none signals to the world that your domain is not expecting strict TLS connections under MTA-STS at that moment.

MTA-STS policy with mode set to 'none'

version: STSv1
mode: none
max_age: 86400
mx: mail.yourdomain.com

Using this mode helps avoid disruptions to email flow if your MTA-STS setup isn't perfect. However, it's crucial to remember that operating in none mode leaves your inbound email susceptible to attacks that MTA-STS is designed to prevent. Therefore, it should only be a temporary state, and you should aim to resolve any underlying issues and transition to a more secure mode as quickly as possible. Regularly verify MTA-STS policy changes to ensure proper functioning.

The 'testing' mode

The 'testing' mode: monitor without enforcing

The testing mode in MTA-STS is invaluable for initial deployment and ongoing monitoring. When the policy mode is set to testing, compliant sending mail servers will fetch and process your MTA-STS policy, but they will not strictly enforce it. Instead, they will attempt to deliver email using secure TLS connections as specified, but if a secure connection cannot be established, they will fall back to sending the email insecurely.

The primary benefit of testing mode is that it allows you to identify and fix any issues with your MTA-STS configuration without disrupting email delivery. During this phase, sending mail servers are configured to send TLS reporting (TLS-RPT) data to an email address specified in your TLSRPT record. These reports provide valuable insights into connection attempts and failures, helping you fine-tune your policy before full enforcement. It's an essential step in a staged rollout.

Before 'testing' mode

No MTA-STS policy: Email transport relies on opportunistic TLS, or no encryption.
Vulnerable to downgrade attacks: Attackers can force unencrypted connections.
No visibility: Difficult to know if secure connections are being used.

During 'testing' mode

Policy fetched, not enforced: Servers gather your MTA-STS policy.
Fallback to insecure delivery: If TLS fails, mail is still delivered.
Receive TLS-RPT reports: Get feedback on TLS connection attempts.

While in testing mode, your domain gains some protection because compliant servers will attempt secure connections first. However, the fallback mechanism means that the strongest security protections are not yet active. It's a temporary, diagnostic state that allows you to confidently move towards full enforcement.

The 'enforce' mode

The 'enforce' mode: full security enforcement

The enforce mode in MTA-STS is the ultimate goal of any MTA-STS deployment. When your policy is set to enforce, compliant sending mail servers will strictly adhere to your policy. This means that if a secure TLS connection cannot be established according to your MTA-STS rules, the email will not be delivered. Instead, it will be deferred or rejected, preventing it from being sent over an insecure channel.

This mode provides the highest level of protection against various email threats, including passive eavesdropping, active attacks, and downgrade attacks. It ensures that all inbound mail from MTA-STS-aware sending domains arrives securely, reinforcing your domain's overall email security posture. Before transitioning to enforce, it's critical to have thoroughly tested your configuration in testing mode and resolved all identified issues.

Potential impact of 'enforce' mode

Security enhancement: Ensures mandatory TLS for email delivery.
Reduced deliverability: If misconfigured, emails may be deferred or rejected.
Prerequisite for DMARC reporting: MTA-STS provides crucial insights for DMARC. DMARC monitoring is vital.

Monitoring remains critical even in enforce mode. You should continue to review TLS-RPT reports to ensure that your policy is consistently being met and that no legitimate email is being blocked. A robust DMARC reporting tool, like Suped, can help you keep track of all your email authentication protocols and provide actionable insights for optimal deliverability.

Policy management and best practices

Implementing an MTA-STS policy involves more than just setting the mode. You also need to define the mx field in an MTA-STS policy specifying which MX hosts are allowed to receive mail securely, and the max_age value, which tells sending servers how long to cache your policy. The format of the MTA-STS policy file itself is a simple YAML-like text file served over HTTPS. This file is discovered via a TXT record in your DNS, making the DNS record type used for MTA-STS policy discovery another critical component.

DNS server and policy document representing MTA-STS policy discovery.

A recommended rollout strategy involves starting with testing mode, closely monitoring TLS-RPT reports, and then gradually transitioning to enforce once you are confident in your setup. The version field in an MTA-STS policy also plays a role, as it helps signal policy updates to sending servers.

Mode	Description	Action on TLS failure	Use case
none	Disables MTA-STS enforcement.	Falls back to opportunistic TLS or no encryption.	Temporarily disabling, issue resolution.
testing	Monitors policy without strict enforcement.	Falls back to insecure, but sends TLS-RPT reports.	Initial deployment, error detection, data collection.
enforce	Strictly enforces MTA-STS policy.	Defers or rejects email, preventing insecure delivery.	Full security protection, production use.

Using a platform like Suped allows you to centralize your email authentication management, including DMARC, SPF, DKIM, and MTA-STS. Our AI-powered recommendations help you interpret TLS-RPT data and fix common DMARC issues and other related security problems, streamlining the process of reaching optimal email deliverability.

Conclusion

The three MTA-STS policy modes—none, testing, and enforce—provide a flexible framework for implementing and managing secure email transport. Each mode has a distinct role, allowing domains to gradually adopt MTA-STS, monitor its performance, and eventually achieve full, strict enforcement.

Careful progression through these modes, coupled with diligent monitoring of TLS-RPT reports, is essential for a successful MTA-STS deployment. This structured approach helps prevent email delivery disruptions while maximizing the security benefits of the protocol. Without proper management, the benefits of MTA-STS cannot be fully realized, and even worse, it could lead to email deliverability issues.

By understanding and correctly utilizing each mode, organizations can significantly strengthen their email security and ensure that sensitive communications are always transmitted over encrypted channels. Tools like Suped provide the necessary visibility and guidance to navigate these complexities, ensuring your email ecosystem remains secure and reliable.