How to detect and verify MTA-STS policy changes and locations?

Key findings

• DNS TXT records: MTA-STS relies on a specific DNS TXT record (e.g., _mta-sts.yourdomain.com) to announce the presence of an MTA-STS policy.
• Policy file location: The actual MTA-STS policy is hosted on a web server at a fixed, well-known URL: https://mta-sts.yourdomain.com/.well-known/mta-sts.txt. This file contains critical policy details like the version, mode (enforce, testing, none), and maximum age.
• Policy ID: The id tag within the policy file acts as a version indicator. Changes to this ID signal updates to the policy, prompting sending servers to re-fetch the policy.
• Monitoring tools: Specialized tools and manual checks of DNS and HTTP endpoints are necessary to verify the correct publication and content of MTA-STS records and policies. For comprehensive monitoring, integrating MTA-STS into your overall DMARC monitoring strategy is advisable, as both contribute to email authentication.

Key considerations

• Impact of changes: Incorrect or unverified MTA-STS policy changes can lead to email delivery issues, as mail servers may reject messages that fail to meet the declared security standards. It's crucial to verify your DNS records diligently.
• Rollback scenarios: Be prepared for situations where a domain might temporarily revert its MTA-STS policy (e.g., from enforce to testing) if issues arise during implementation, as observed with major email providers. This requires continuous vigilance.
• Log analysis: Analyzing your mail server logs can provide real-time insights into how external domains are interpreting and applying your MTA-STS policy. Look for increased logging of enforce policies to confirm successful adoption.
• Policy expiration: MTA-STS policies have a maximum age, typically up to 6 months. Sending email services regularly check the policy ID for changes, so ensure your policy is always current and accessible.

Key opinions

• Detection methods: Marketers frequently inquire about the best ways to detect MTA-STS policy changes, often relying on log analysis from their sending infrastructure.
• Error monitoring: A common concern is whether policy shifts lead to errors or delivery failures, emphasizing the need for robust error reporting and monitoring systems.
• Policy location awareness: Understanding where the MTA-STS policy file is located (e.g., the .well-known/mta-sts.txt path) is essential for direct verification.
• Real-time updates: Marketers value timely updates on changes, especially from major inbox providers, to proactively adjust their strategies.

Key considerations

• Impact on deliverability: Incorrect MTA-STS setup or unverified policy changes can inadvertently lead to email delivery failures, emphasizing the importance of careful implementation.
• Policy testing: Initially deploying MTA-STS in testing mode is often recommended to gather TLS reports and monitor behavior before moving to enforce.
• DNS propagation: Changes to the MTA-STS DNS TXT record, like other email authentication DNS records, require time for DNS propagation across the internet before they are universally recognized.
• Avoiding errors: Marketers should be aware of common pitfalls such as failure to resolve the policy host or incorrect policy file configuration, which can hinder secure email delivery. Checking the policy regularly, for instance via a DNS lookup tool, is a practical step.

Key opinions

• Policy enforcement observation: Experts note when large domains like Yahoo shift their MTA-STS policy to enforce mode, indicating a move towards stricter TLS requirements for inbound mail.
• Detection via logs: Detection of policy changes often comes from observing an increase in messages logged as subject to an enforce policy in mail transfer agent (MTA) logs, even without explicit errors.
• Fixed policy location: Experts emphasize that the MTA-STS policy is always found at a standardized, fixed URL (e.g., https://mta-sts.domain.com/.well-known/mta-sts.txt), which is key for programmatic checks.
• Policy rollbacks: It is not uncommon for domains to temporarily revert their policy mode (e.g., from enforce to testing) if unexpected issues or compatibility problems arise.

Key considerations

• Interoperability: Ensure your mail infrastructure (like Exchange 2019 or Exchange Online) is configured to support MTA-STS, as this directly impacts your ability to send and receive mail securely according to the policy.
• DNS and web hosting for policy: The MTA-STS TXT record must be correctly published in DNS, and the policy file must be hosted on a web server accessible via HTTPS. This setup is critical for successful implementation.
• Monitoring for policy adherence: Beyond initial setup, continuous monitoring of both your own and recipient domains' MTA-STS policies is vital. This proactive approach helps identify and address any potential deliverability impacts related to secure transport, much like monitoring DMARC reports.
• Debugging policy issues: When encountering issues, verify both the DNS TXT record and the HTTP endpoint for the policy file. Check for common problems like DNS resolution failures or incorrect policy file content, which can be identified using various online checking tools.

Key findings

• Standardized location: Documentation specifies that the MTA-STS policy file must be served over HTTPS from a fixed, well-known path within the mta-sts subdomain (e.g., https://mta-sts.example.com/.well-known/mta-sts.txt).
• Policy file contents: The policy file itself is a plain text file containing key-value pairs, including version, mode, mx records, and max_age.
• DNS TXT record: A DNS TXT record for _mta-sts.yourdomain.com must contain v=STSv1 and an id tag, which is crucial for signaling policy updates.
• Policy update mechanism: The id tag is used by sending servers to determine when the policy has been updated. A change in the id value signals a need to re-fetch the policy.

Key considerations

• Policy modes: Documentation specifies three modes: enforce, testing, and none. It is generally recommended to start with testing to analyze TLS Reports before enforcing the policy.
• Max_age parameter: The max_age tag defines how long sending mail servers can cache the policy. It is critical to manage this value, ensuring that changes propagate within a reasonable timeframe.
• Verification process: Documentation outlines methods for verifying the policy, such as directly accessing the mta-sts.txt file via HTTPS to confirm its content and accessibility.
• Compatibility: It is important to understand which mail services and versions support MTA-STS (e.g., Exchange Online support) to ensure your implementation is effective for your target recipients. For related authentication, consider how this interacts with SPF, DKIM, and DMARC efforts.