How long should I stay in MTA-STS testing mode?

The duration depends on your mail volume and the complexity of your infrastructure. It's generally recommended to stay in testing mode for several weeks, or until you consistently receive clean reports indicating no connection failures for legitimate email traffic. This allows sufficient time for various sending MTAs to fetch and test your policy.

What kind of reports do I get in testing mode?

In testing mode, sending MTAs that support MTA-STS (and DMARC reporting) will send aggregate reports (RUA) detailing connection attempts to your domain. These reports will highlight any failures to establish secure connections, TLS certificate issues, or policy mismatches, helping you identify and fix problems.

Can I skip testing mode and go straight to enforce mode?

While technically possible, skipping testing mode is strongly discouraged. Deploying directly to 'enforce' mode without prior testing carries a significant risk of legitimate emails being rejected or delayed if your MTA-STS configuration or mail server setup has any issues. Testing mode is crucial for a smooth transition.

What happens if a sending server doesn't support MTA-STS?

If a sending server does not support MTA-STS, it will simply ignore your MTA-STS policy. Email delivery will proceed as usual, without the added security layer of MTA-STS. However, it will still rely on other email authentication standards like DMARC, SPF, and DKIM for security.

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

An illustration showing a magnifying glass inspecting email server connections, representing MTA-STS testing mode.

MTA-STS (Mail Transfer Agent Strict Transport Security) is a crucial standard designed to enhance the security of email communications by ensuring that emails are sent over secure, encrypted connections. It helps prevent downgrade attacks and Man-in-the-Middle (MITM) attacks during email transit, protecting your messages from eavesdropping and tampering.

The core of an MTA-STS policy lies in its 'mode' field, which dictates how sending mail servers should treat connections to your domain. This field is vital because it determines the level of enforcement and reporting. Understanding the various modes, especially 'testing' mode, is fundamental to a successful and secure MTA-STS deployment.

Testing mode allows domain owners to deploy an MTA-STS policy without immediately enforcing strict security rules. Instead, it enables monitoring and reporting, providing valuable insights into potential issues without disrupting mail flow. It is a critical initial step for verifying your setup before moving to full enforcement.

Understanding MTA-STS modes

MTA-STS policies come with three primary modes: 'none', 'testing', and 'enforce'. Each mode serves a distinct purpose in the lifecycle of deploying this email security standard. To fully grasp the purpose of testing mode, it helps to understand its counterparts.

The 'none' mode effectively disables MTA-STS enforcement. It instructs sending servers to treat your domain as if it does not have an MTA-STS policy, offering no additional security. The 'enforce' mode, conversely, is the most stringent. It mandates that all incoming email connections must adhere to your specified policy, failing to deliver mail if secure connections cannot be established. You can learn more about what 'enforce' mode means in MTA-STS and what 'none' mode entails.

The 'testing' mode bridges the gap between no enforcement and full enforcement. As we have discussed the 'mode' field in an MTA-STS policy, 'testing' mode allows external mail servers to fetch your policy and identify any connection issues without rejecting mail. It's a probationary period where you can fine-tune your configuration and resolve problems based on the feedback received, ensuring a seamless transition to a fully enforced policy.

Benefits of testing mode

The primary advantage of testing mode is its ability to provide a safe environment for deployment. It lets you validate your MTA-STS setup thoroughly. This ensures that your mail servers are correctly configured to support TLS and that your policy file is accessible and accurate without risking legitimate email delivery.

During testing, external mail servers that support MTA-STS will attempt to connect securely to your domain based on your published policy. If they encounter issues, such as certificate mismatches or unsupported TLS versions, they will still deliver the mail over a less secure connection (or fallback to MX records if the policy isn't strict). Crucially, they also generate daily reports detailing any detected problems. These reports are invaluable for identifying and resolving configuration errors. Google provides more information about these daily reports.

This preventative approach significantly mitigates the risk of mail flow disruptions. By monitoring these reports, you can make necessary adjustments, like ensuring your MTA-STS policy file format is correct, before switching to 'enforce' mode. This guarantees that when you finally enforce your policy, your email deliverability remains uncompromised, and your communications are genuinely secured.

Why start with testing mode?

Risk mitigation: Avoids accidental email blocking or delayed delivery due to misconfigurations.
Feedback mechanism: Provides detailed reports on connection failures, enabling quick fixes.
Validation: Confirms that your mail servers support secure TLS connections and your policy is correctly published.
Gradual deployment: Allows for a smooth, controlled transition to full security enforcement.

Implementing MTA-STS in testing mode

Implementing MTA-STS in testing mode involves a few key steps. First, you need to create an MTA-STS policy file. This file, often named mta-sts.txt, is hosted on a well-known URL path on your domain, specifically https://mta-sts.yourdomain.com/.well-known/mta-sts.txt. The file name for an MTA-STS policy is important, so ensure it matches. This file declares your policy, including the 'mode' (set to 'testing'), the 'version' field, and the MX records authorized to receive mail for your domain.

Next, you publish a DNS TXT record for your domain. This record signals to sending mail servers that your domain supports MTA-STS and specifies the ID of your current policy. This DNS record must be published at _mta-sts.yourdomain.com. The purpose of this MTA-STS TXT record is to notify other mail servers that they should check for your MTA-STS policy file. Microsoft Learn explains more about enhancing mail flow with MTA-STS.

Example MTA-STS policy file (mta-sts.txt) in testing modeplain

version: STSv1
mode: testing
mx: mail.yourdomain.com
mx: *.yourdomain.com
max_age: 86400

Example DNS TXT record for MTA-STSplain

_mta-sts.yourdomain.com. IN TXT "v=STSv1; id=20240101T120000;"

An illustration of secure email connections being verified by a policy document and magnifying glass.

After deploying these, it's crucial to actively monitor the reports generated by sending mail servers. These reports, often delivered as DMARC Aggregate Reports (RUA), contain detailed information about connections and any failures encountered. Analyzing these reports is key to identifying and resolving configuration issues. Platforms like Suped offer robust DMARC monitoring and can help simplify this process by providing actionable recommendations based on your aggregated data, including insights relevant to MTA-STS implementation. This ensures you can detect and verify MTA-STS policy changes effectively.

Transitioning from testing to enforce mode

Once you've consistently received clean reports (meaning no failures or unexpected fallbacks) over a sufficient period, typically a few weeks to a month, you are ready to transition your MTA-STS policy from 'testing' to 'enforce' mode. This confidence comes from knowing that all legitimate email traffic can establish secure connections without issues.

Testing mode characteristics

Policy enforcement: No strict enforcement, allows fallback to less secure connections.
Monitoring: Generates reports (e.g., via DMARC RUA) on connection issues.
Risk level: Low risk of mail delivery failure.

Enforce mode characteristics

Policy enforcement: Strict enforcement, requires secure connections, fails if not met.
Monitoring: Continues to generate reports, but with stronger implications for failures.
Risk level: Higher risk of mail delivery failure if misconfigured.

Field	Testing mode example	Enforce mode example
mode	mode: testing	mode: enforce
max_age	max_age: 86400 (1 day recommended for testing)	max_age: 604800 (7 days or more for enforce)

The transition itself is simple: you update the 'mode' field in your MTA-STS policy file from 'testing' to 'enforce', and increment the 'id' in your DNS TXT record to signal the policy change. This signals to sending MTAs that your domain is now enforcing strict transport security, and they should only deliver mail over encrypted channels that meet your policy requirements.

Final thoughts on MTA-STS testing mode

The 'testing' mode in MTA-STS is more than just a preliminary step; it's an essential safety net and diagnostic tool in the journey toward full email transport security. It allows for meticulous verification of your setup, provides critical feedback through reports, and prevents service disruption during deployment. By diligently using testing mode, organizations can confidently move to 'enforce' mode, ensuring their email communications are safeguarded against modern threats.