What is the standard value for the 'version' field in an MTA-STS policy?

The standard and currently mandated value for the 'version' field is STSv1 . This value is defined by the RFC 8461 standard and must be the first line in your MTA-STS policy file.

What happens if the 'version' field is missing or incorrect?

If the 'version' field is missing or contains an incorrect value, sending mail transfer agents (MTAs) may fail to parse your MTA-STS policy. This could lead to the policy being ignored, emails being sent without encryption, or in 'enforce' mode, even being rejected, thereby compromising your email security and deliverability.

How does the 'version' field relate to the 'id' tag in the DNS TXT record?

The 'version' field in the MTA-STS policy file specifies the policy's standard. The 'id' tag in the MTA-STS policy TXT record is a unique identifier that signals when the policy has been updated. While both relate to policy versions, the 'version' field itself defines the syntax and semantics within the policy file, while the 'id' tag triggers discovery of policy changes.

Can I use a different version for MTA-STS in the future?

Currently, STSv1 is the only defined version. However, the inclusion of a 'version' field implies that future revisions like STSv2 could be introduced. This design allows for forward compatibility, enabling domains to update their policies to new standards while potentially maintaining backward compatibility.

What is the 'version' field in an MTA-STS policy? - MTA-STS - Email authentication - Knowledge base

Q: What is MTA-STS and why is it important?

MTA-STS (Mail Transfer Agent Strict Transport Security) is an email security standard that enforces TLS encryption for SMTP connections. It helps prevent downgrade attacks and man-in-the-middle attacks, ensuring that emails sent to your domain are always encrypted, thus protecting sensitive information from interception or tampering during transit.

Two mail servers securely exchanging emails, emphasizing the MTA-STS policy's version field.

When you implement MTA-STS (Mail Transfer Agent Strict Transport Security) for your domain, you're setting a clear standard for how other mail servers should connect with yours. This crucial email security protocol helps prevent downgrade attacks and man-in-the-middle attacks by ensuring that all incoming SMTP connections are encrypted using Transport Layer Security (TLS).

At the heart of this security mechanism is the MTA-STS policy file. This file contains various directives that dictate how your mail servers should behave. Among these directives, one of the most fundamental is the 'version' field. It's often overlooked, but its correct configuration is paramount for your policy to be interpreted and enforced effectively by sending MTAs worldwide.

The critical role of the version field

The 'version' field, typically designated as version: STSv1, is the very first line you should see in an MTA-STS policy file. Its primary purpose is to declare which version of the MTA-STS standard your policy adheres to. This is crucial because it tells any connecting mail server how to parse and understand the rules you've laid out. Without this field, or with an incorrect value, a sending MTA may not be able to correctly process your policy, leading to potential security vulnerabilities or delivery failures.

The RFC 8461 standard, which defines MTA-STS, mandates that the version field must be present and set to STSv1 for current implementations. This ensures that all compliant mail servers operate on the same understanding of the policy structure and directives. It's similar to how an HTML document declares its version to ensure web browsers render it correctly, except here the stakes are much higher, involving the security of email communications.

While STSv1 is the only currently defined version, the existence of a 'version' field anticipates future revisions of the MTA-STS standard. Should STSv2 or later versions be introduced, this field would allow your domain to upgrade its policy without breaking compatibility with older mail servers that still understand STSv1. It's a forward-thinking design choice to ensure long-term flexibility and evolution of the protocol.

The journey of an MTA-STS policy begins with a DNS record type that announces your domain uses MTA-STS. This TXT record contains a 'version' tag (e.g., v=STSv1) and an 'id' tag. This initial 'v' tag is distinct from the 'version' field within the policy file itself, but both signal the protocol version in use and are critical for proper policy discovery and application.

Understanding the MTA-STS policy file format

The MTA-STS policy file is a simple text file, often named mta-sts.txt, that you host on your web server at a specific HTTPS path. You can learn more about the file name for an MTA-STS policy if you are setting one up. The file uses a key-value pair format, with each directive on a new line. As mentioned, the 'version' field must always be the first entry to ensure it's processed correctly before any other policy rules.

This strict ordering is by design. A sending MTA needs to immediately identify the policy version to know how to interpret the subsequent fields. If the 'version' field is misplaced or malformed, the entire policy could be deemed invalid, and the MTA might revert to less secure email delivery methods or even reject the email. This is why paying close attention to the format is essential.

Example MTA-STS policy fileyaml

version: STSv1
mode: enforce
max_age: 604800
mx: mail.example.com
mx: *.mail.example.net

Beyond the 'version' field, an MTA-STS policy typically includes other critical directives such as the 'mode' field (e.g., 'enforce', 'testing', or 'none'), 'max_age' (how long the policy is cached), and 'mx' records (listing the mail exchange servers for your domain). Each of these fields plays a specific role in securing email transmission, but they all depend on the foundational 'version' field for proper interpretation. For more details on these fields, refer to the official RFC 8461 standard.

Adhering to policy format

Always ensure the 'version' field is the first line in your MTA-STS policy file. Incorrect placement or syntax can render your entire policy invalid, leaving your email communications vulnerable to unencrypted delivery or attacks.

Why versioning matters for secure email delivery

Correct versioning ensures that sending mail transfer agents (MTAs) interpret your MTA-STS policy as you intended. When an MTA receives your policy, it first checks the 'version' field. If the version is recognized (i.e., STSv1), it then proceeds to apply the 'mode', 'max_age', and 'mx' rules defined in the policy. This consistent interpretation prevents misconfigurations that could inadvertently lead to less secure or even unencrypted email connections.

The risks of an incorrect or missing 'version' field are significant. If a sending MTA cannot parse the policy due to a faulty version, it might default to its own local policy or, worse, attempt an unencrypted SMTP connection. This undermines the very purpose of MTA-STS, which is to enforce TLS encryption. In 'enforce' mode, a policy parse error could even result in email rejection, impacting your deliverability.

Valid version field

Policy recognized: Sending MTAs correctly identify and apply the STSv1 standard.
Enforced TLS: All incoming connections are properly encrypted according to your policy's mode.
Consistent security: Your domain maintains a robust security posture for email reception.

Invalid version field

Policy ignored: Sending MTAs may discard your policy due to a parsing error.
Insecure connections: Emails might be sent over unencrypted channels, or fall back to SMTP.
Deliverability impact: In 'enforce' mode, misconfigured policies can lead to emails being rejected.

Consistent interpretation across all mail servers is the cornerstone of effective transport layer security. By correctly specifying version: STSv1, you are ensuring that your domain's security preferences are universally understood and respected, thus providing a critical layer of protection for inbound email traffic. This helps mitigate risks like eavesdropping and tampering during transit.

Implementing and validating your version field

When you're setting up your MTA-STS policy, the process of defining the 'version' field is straightforward, but it's crucial to get it right. You simply need to include version: STSv1 as the very first line of your mta-sts.txt file. After you've published your policy, it's not enough to simply set it and forget it. Ongoing monitoring is essential to ensure that your policy remains valid and effective.

Additionally, whenever you make changes to your MTA-STS policy, even minor ones like adjusting the 'max_age' or 'mode' fields, you must also update the 'id' tag in your MTA-STS policy TXT record. This 'id' acts as a serial number, signaling to sending MTAs that a new version of your policy is available and needs to be fetched. Failing to update the 'id' means older policies might continue to be cached and applied, effectively negating your changes until the previous 'max_age' expires.

A magnifying glass inspecting an MTA-STS policy, highlighting the correct version field for validation.

The entire process of detecting and verifying MTA-STS policy changes involves several steps, from DNS queries to fetching the policy file over HTTPS. Each stage relies on accurate configuration. Tools that provide comprehensive DMARC monitoring and reporting can also help you keep an eye on your MTA-STS status, offering insights into whether your policies are being correctly applied by inbound mail servers. This ensures your efforts in enhancing email security are paying off.

Ensuring robust MTA-STS deployment

The 'version' field in your MTA-STS policy may seem like a small detail, but it's the foundational element that dictates how other mail servers interpret your domain's commitment to secure email transport. Correctly setting it to STSv1 and ensuring it's the first line in your policy file is critical for successful implementation and enforcement of MTA-STS.

By understanding and meticulously configuring the 'version' field, alongside other policy directives like 'mode' and 'max_age', you significantly strengthen your domain's email security posture. This proactive approach helps protect your inbound email traffic from interception and tampering, contributing to a more trusted and secure communication ecosystem.

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Field	Description	Required Status
version	Indicates the version of the MTA-STS policy standard, currently STSv1.	Mandatory
mode	Specifies the enforcement behavior: 'enforce', 'testing', or 'none'.	Mandatory
max_age	The duration (in seconds) that the policy should be cached by sending MTAs. From the IANA assignments.	Mandatory
mx	A list of MX hostnames that are authorized to receive mail for the domain, wildcard accepted.	Mandatory (at least one)