What is the purpose of the 'id' tag in an MTA-STS policy TXT record?
Matthew Whittaker
Co-founder & CTO, Suped
Published 10 Jun 2025
Updated 2 Nov 2025
8 min read
When we talk about strengthening email security, MTA-STS (Mail Transfer Agent Strict Transport Security) is a crucial protocol that ensures email servers communicate over secure, authenticated TLS connections. At the heart of MTA-STS policy management lies a seemingly small but incredibly important component: the 'id' tag in the MTA-STS policy TXT record.
This tag acts as a version indicator for your MTA-STS policy, playing a vital role in how sending email servers (known as MTAs) determine if your domain's security policy has been updated. Without it, ensuring that all mail exchanges adhere to your latest security directives would be significantly more complex and error-prone. Understanding its function is key to maintaining robust email security.
The 'id' tag ensures that when you modify your MTA-STS policy, recipient mail servers are properly notified to fetch the new rules. This prevents them from applying outdated security configurations, which could lead to email delivery issues or, more critically, expose your email communications to security risks.
The critical role of the 'id' tag
The role of the 'id' tag
The 'id' tag serves as a version identifier within your MTA-STS DNS TXT record. Its primary purpose is to signal to sending Mail Transfer Agents that the MTA-STS policy has changed and needs to be re-fetched. When a sending MTA wants to deliver email to your domain, it first checks your DNS for a specific MTA-STS TXT record. This record contains various parameters, and the 'id' tag is one of them. For a detailed technical specification of MTA-STS, you can refer to the official RFC 8461 document.
Without this versioning mechanism, a sending MTA would have no reliable way to know if the policy it previously cached is still current. It would either have to frequently check for updates, which is inefficient, or risk using an outdated policy, which could undermine the security benefits of MTA-STS. This small 'id' tag therefore ensures efficiency and security.
Think of it like a software version number. When the version number changes, you know there's a new update available. Similarly, when the 'id' tag value in your MTA-STS TXT record changes, it tells the world that a new policy file has been published. The 'id' tag is required in the TXT record, alongside the 'v' tag which indicates the policy version (currently only STSv1). This mechanism is why the 'id' is so fundamental to the proper functioning of MTA-STS.
Policy discovery and updates
How the 'id' tag facilitates policy updates
When a sending MTA prepares to send an email, it performs a series of checks. First, it looks for the presence of the MTA-STS TXT record in your DNS. If found, it extracts the 'id' value from this record. It then compares this 'id' with any previously cached 'id' for your domain. If the 'id' values differ, the sending MTA understands that a new MTA-STS policy has been published.
Upon detecting a new 'id', the sending MTA will then fetch the updated policy file from your web server, which should be hosted at a specific MTA-STS policy file location. This ensures that the most current security rules, including accepted MX hosts (see the 'mx' field) and the 'mode' field, are applied to all subsequent email deliveries. It is a critical handshake in the secure email ecosystem.
Important for MTA-STS configuration
The id attribute must be updated in the MTA-STS Discovery TXT Record whenever the MTA-STS policy file is changed. This ensures that sending MTAs are aware of and retrieve the most current policy, maintaining the integrity and security of your email flow. Forgetting to update it can cause significant deliverability and security issues.
This policy retrieval process is standardized. Microsoft's documentation on MTA-STS highlights how the 'id' tag indicates support to a sender, prompting them to retrieve the HTTPS-based policy. You can read more about enhancing mail flow with MTA-STS to understand the full scope of its benefits.
Risks of neglecting the 'id' tag
Consequences of an outdated 'id' tag
Failing to update the 'id' tag after modifying your MTA-STS policy can lead to serious problems. Sending MTAs that have previously cached your old 'id' will continue to operate under your old policy, even if you've made critical security updates. This means that if you've changed your accepted MX servers (using the 'mx' field) or altered your enforcement mode (the 'mode' field in an MTA-STS policy), these changes won't be universally recognized.
The primary risk is a potential security bypass. If your old policy allowed less secure connections or permitted mail flow to servers that are no longer authorized, an outdated 'id' tag means some email traffic could still flow through those less secure channels. This defeats the very purpose of implementing MTA-STS, leaving you vulnerable to downgrade attacks and man-in-the-middle attacks.
Updated MTA-STS Policy
Secure connections: All inbound email traffic is enforced to use secure TLS connections based on the latest policy.
Preventing attacks: Protection against man-in-the-middle and downgrade attacks is active and current.
Deliverability: Email is delivered smoothly to the intended mail servers, adhering to correct MX records.
Outdated MTA-STS Policy
Security vulnerabilities: Emails may still be sent over unencrypted connections if allowed by the old policy.
Policy enforcement failures: New security directives are not applied, leaving the domain exposed.
Misdirected mail: Email might be sent to incorrect or deprecated mail servers.
Beyond security, an outdated 'id' can cause email deliverability issues. If you update your MX records (the 'mx' rule in MTA-STS) but fail to update the 'id' tag, some sending MTAs might still attempt to deliver email to your old, now invalid, MX hosts. This can result in delayed emails, bounces, or even lost mail, directly impacting communication and business operations. It’s crucial to treat the 'id' tag as an integral part of your policy's lifecycle.
Managing your 'id' tag effectively
Best practices for managing the 'id' tag
To effectively manage your MTA-STS 'id' tag, I recommend adopting a systematic approach. The most common and highly recommended format for the 'id' value is a date-based string, such as YYYYMMDDHHMMSS (e.g., 20251018153000). This provides a clear, chronological indicator of when the policy was last updated, making it easy to track changes and troubleshoot if necessary.
Always update the 'id' tag whenever you make any change to your MTA-STS policy file. This includes modifying 'mode', 'mx', or 'max_age' values. Consistency in updating the 'id' ensures that all legitimate mail servers fetching your policy will receive the latest rules, preventing the use of stale or insecure configurations. Mimecast also emphasizes the importance of updating the 'id' attribute in the discovery TXT record after any policy file publication. Their documentation highlights this practice.
Example MTA-STS TXT Record with 'id' tagdns
_mta-sts.yourdomain.com. IN TXT "v=STSv1; id=20240722103000;"
Integrating MTA-STS management with your broader email security strategy is also beneficial. While MTA-STS secures the transport of email, protocols like DMARC (Domain-based Message Authentication, Reporting, and Conformance) protect against email spoofing and phishing by verifying the sender's authenticity. A robust DMARC monitoring solution can provide visibility into both authentication failures and successful deliveries, helping you identify and resolve issues promptly. Tools like Suped can offer AI-powered recommendations to streamline these processes and ensure your 'id' tags and policies are always optimized.
For ongoing vigilance, leverage DMARC reporting platforms that offer real-time alerts. These systems, like Suped, can notify you immediately of any changes in your email authentication landscape or potential misconfigurations. They also provide SPF flattening to address the 10-lookup limit and a unified dashboard for DMARC, SPF, and DKIM monitoring, alongside blocklist monitoring. This comprehensive approach ensures that your email infrastructure is not only secure but also highly reliable, maintaining good sender reputation and deliverability.
Conclusion
Ensuring secure email with MTA-STS and the 'id' tag
The 'id' tag in an MTA-STS policy TXT record is a small but mighty component in the email security landscape. It acts as the version control for your MTA-STS policy, ensuring that sending mail servers are always aware of and adhering to your latest security configurations. Proper management of this tag is fundamental to preventing security vulnerabilities and maintaining consistent email deliverability.
By following best practices, such as using date-based IDs and integrating MTA-STS monitoring with your broader email authentication strategy, you can significantly enhance your domain's email security posture. Tools like Suped can simplify the complexity of DMARC, SPF, DKIM, and MTA-STS, providing the insights and automation needed to keep your email communications safe and reliable.
