What is the specific name of the MTA-STS DNS TXT record?

The MTA-STS DNS TXT record is published at the subdomain _mta-sts . So, for a domain like example.com , the full record name would be _mta-sts.example.com .

What information does the MTA-STS TXT record contain?

The TXT record contains the protocol version, v=MTA-STS v1 , and an id tag (e.g., id=20231026103000 ). The id tag acts as a version indicator for the MTA-STS policy file.

Why is the 'id' tag in the MTA-STS TXT record important?

The id tag signals to sending MTAs when a domain's MTA-STS policy has been updated. When this id changes, it prompts the sending MTA to fetch the new policy file from the specified HTTPS location, ensuring they use the latest security rules.

Does MTA-STS require any other DNS records besides the TXT record?

While the TXT record initiates policy discovery, MTA-STS also uses a corresponding policy file hosted on a web server via HTTPS. This file, typically named mta-sts.txt , is fetched after the TXT record is discovered. There is also typically a DNS TXT record for TLS-RPT (SMTP TLS Reporting) which allows domains to receive reports on MTA-STS policy enforcement.

What DNS record type is used for MTA-STS policy discovery? - MTA-STS - Email authentication - Knowledge base

Email security is a complex landscape, constantly evolving to combat sophisticated threats. While protocols like SPF and DKIM address sender authentication, ensuring secure transport of email between servers remains a critical challenge. This is where Mail Transfer Agent Strict Transport Security (MTA-STS) comes into play, providing a mechanism to enforce TLS encryption for SMTP connections.

MTA-STS helps prevent downgrade attacks and Man-in-the-Middle (MitM) attacks by ensuring that a sending server (MTA) only delivers email to a receiving server if a secure, authenticated TLS connection can be established. Without MTA-STS, an attacker could strip away TLS encryption, forcing email to be sent over an unencrypted channel, or redirect mail to a malicious server.

The foundation of MTA-STS lies in how a sending server discovers the receiving domain's security policy. This discovery process is crucial for the protocol to function effectively. It requires a specific DNS record type to advertise the presence and location of the MTA-STS policy.

Understanding this foundational element is key to implementing and managing robust email security measures for your domain.

The TXT record: signaling MTA-STS availability

The DNS record type used for MTA-STS policy discovery is a TXT record. This is a common record type used for various text-based information within DNS, such as DMARC, SPF, and DKIM. Specifically, the MTA-STS TXT record signals to sending MTAs that your domain supports MTA-STS and indicates where its policy file can be found. You can learn more about the purpose of the MTA-STS TXT record in our dedicated guide.

This record is published on a special subdomain: _mta-sts. For example, if your domain is example.com, the record would be located at _mta-sts.example.com. This standardized naming convention ensures that mail servers know exactly where to look for the MTA-STS policy.

The value of this TXT record contains a single key-value pair, v=MTA-STS v1, to declare the protocol version, and an id tag. This id tag is a randomly generated string, typically a timestamp, that acts as a version indicator for your policy. Each time you update your MTA-STS policy file, you must update the id in this TXT record. This tells receiving MTAs that a new policy is available and needs to be fetched, ensuring they always have the most current security rules. For further details, consult the RFC 8461 specification.

Example MTA-STS TXT recordDNS

_mta-sts.example.com. IN TXT "v=MTA-STS v1; id=20231026103000"

Understanding the MTA-STS policy discovery process

When a sending email server wants to send mail to a domain, it first queries the DNS for an MTA-STS TXT record for that domain. If such a record is found, it indicates that the domain has an MTA-STS policy in place. The id value in the TXT record helps the sending server determine if it needs to fetch a new policy or if its cached policy is still current. This versioning is vital for efficient policy updates without requiring immediate DNS propagation.

Upon discovering the TXT record, the sending server then attempts to retrieve the actual MTA-STS policy file. This policy file is hosted on a web server over HTTPS, typically at a well-known URL like https://mta-sts.example.com/.well-known/mta-sts.txt. The policy file itself is a YAML-like text file that specifies the domain's email exchange servers (MX records) and the required TLS enforcement mode. We have a detailed guide on what is the format of the MTA-STS policy file.

This two-step process, involving both a DNS TXT record and an HTTPS-served policy file, provides a robust and verifiable method for domains to declare their email transport security preferences. It is important to remember that the HTTPS server hosting the policy must present a valid, trusted TLS certificate for the policy to be considered legitimate.

Protocol	DNS record type	Purpose
MTA-STS	TXT	Signals MTA-STS support and points to the policy file.
DMARC	TXT	Publish domain's DMARC policy for email authentication.
SPF	TXT	Lists authorized sending IP addresses and domains.
DKIM	TXT (or CNAME)	Contains the public key for verifying email signatures.
BIMI	TXT	Advertises brand logos in supporting email clients.

Ensuring proper MTA-STS policy deployment

The correct configuration of your MTA-STS TXT record is paramount. An improperly configured record or a mismatch between the id in the TXT record and the actual policy file can lead to emails failing to be delivered or being delivered without the intended TLS protection. This could potentially expose sensitive communications to interception.

While MTA-STS does not directly rely on DNSSEC for policy discovery, having DNSSEC enabled for your domain is a best practice that adds an extra layer of security by protecting your DNS records from tampering. This ensures that the TXT record a sending server fetches is authentic and hasn't been maliciously altered. Microsoft recommends DNSSEC for enhanced mail flow with MTA-STS.

Regularly verifying your MTA-STS configuration, including the TXT record and the policy file, is essential. Tools and services that monitor these configurations can alert you to any discrepancies or issues that might compromise your email transport security. This proactive approach helps maintain trust and deliverability.

Monitoring and advanced insights

Even with correct initial setup, the dynamic nature of DNS and web hosting means that MTA-STS policy changes or issues can arise. Effective monitoring is crucial to ensure continuous protection. This includes keeping an eye on your DNS TXT record's id tag, ensuring it's updated whenever your policy file changes. Without proper vigilance, your domain could inadvertently expose emails to insecure transport.

Monitoring MTA-STS with Suped

For comprehensive monitoring of MTA-STS and other email authentication protocols, Suped offers an advanced platform. Our AI-powered recommendations help you quickly identify and fix issues with your MTA-STS TXT records and policy files. You'll receive real-time alerts about misconfigurations, ensuring your email transport remains secure and compliant.

Our unified platform brings together DMARC monitoring, SPF, and DKIM insights with blocklist and deliverability monitoring, providing a holistic view of your email ecosystem. For MSPs, our multi-tenancy dashboard simplifies managing multiple client domains effectively.

Beyond basic monitoring, understanding the nuances of how MTA-STS interacts with other email authentication protocols is vital for a robust security posture. A strong MTA-STS implementation works in concert with DMARC, SPF, and DKIM to create a comprehensive defense against various email-borne threats.

This layered approach is critical in today's threat landscape. By ensuring all your email authentication mechanisms are correctly configured and consistently monitored, you significantly enhance your domain's reputation and protect your email communications from compromise.

Conclusion

The DNS TXT record is the cornerstone for MTA-STS policy discovery, serving as the initial signal for domains that support this crucial email transport security protocol. Its correct configuration, along with the corresponding policy file, ensures that email is exchanged securely over authenticated TLS connections, safeguarding sensitive information from various cyber threats.

Maintaining the integrity of this TXT record, particularly the id tag, is vital for proper policy updates and continuous security. Proactive monitoring and adherence to best practices are key to leveraging MTA-STS effectively and protecting your email communications in the long term.

As email remains a primary communication channel, adopting robust security standards like MTA-STS is no longer optional but a necessity. By understanding and correctly implementing the underlying DNS mechanisms, you contribute significantly to a more secure email ecosystem for your organization and your recipients.