What is the primary benefit of MTA-STS for inbound email?

The primary benefit is enforcing secure TLS encryption for all incoming mail, protecting against downgrade attacks and ensuring that only authenticated mail servers can deliver email to your domain.

How does my email server use MTA-STS for outbound email?

Your email server, as a sender, can check for and honor the MTA-STS policies of recipient domains. This ensures that your outbound emails are also delivered over secure, authenticated TLS connections to those domains.

Does MTA-STS replace other email authentication standards like DMARC?

No, MTA-STS does not replace other standards. It complements them by securing the transport layer (SMTP over TLS), while DMARC, SPF, and DKIM focus on sender authentication and domain spoofing prevention. They work together for comprehensive email security.

What happens if a domain doesn't have an MTA-STS policy?

If a domain doesn't have an MTA-STS policy, email servers will fall back to traditional SMTP behavior, attempting opportunistic TLS. This means there's no enforcement of encryption, leaving communications vulnerable to downgrade attacks.

Is MTA-STS mandatory for email security?

MTA-STS is not mandatory, but it's highly recommended as a best practice for enhancing email security. It adds an important layer of protection against interception and tampering of emails in transit, for both sending and receiving domains.

Does MTA-STS apply to inbound or outbound mail? - MTA-STS - Email authentication - Knowledge base

Two abstract servers with a secure connection, representing MTA-STS for both inbound and outbound email.

MTA-STS (Mail Transfer Agent Strict Transport Security) is a crucial email security standard designed to enforce encryption (TLS) for email connections. It addresses a vulnerability where attackers could downgrade or intercept email communications that were supposed to be encrypted, often through man-in-the-middle attacks. By publishing an MTA-STS policy, domains can declare their readiness to receive email only via secure channels, making it harder for malicious actors to snoop on or tamper with messages in transit.

The core question often arises whether MTA-STS applies solely to emails coming into a domain or also to emails sent out from a domain. The answer is that MTA-STS is designed to enhance security for both inbound and outbound mail flows, playing a dual role in protecting email communications. Its implementation helps secure your domain's communications regardless of whether it is the sender or the receiver.

To fully leverage MTA-STS, it's essential to understand its mechanisms for each direction of mail flow and ensure proper configuration. This layered approach to email security, alongside other standards like DMARC, SPF, and DKIM, creates a more robust defense against email-based threats.

Inbound mail protection with MTA-STS

For inbound mail, MTA-STS protects your domain by telling other sending mail servers that they should only send email to your domain using a secure, authenticated TLS connection. When a sender's mail server attempts to deliver an email to your domain, it first checks if your domain has an MTA-STS policy published. This policy, retrieved over HTTPS, specifies your domain's legitimate mail exchange (MX) hosts and mandates TLS encryption.

If a policy is found, the sending server will verify the identity of your mail server using its TLS certificate. Should the certificate be invalid or if a secure connection cannot be established, the sending server is instructed to temporarily queue the email and retry later, rather than delivering it insecurely. This mechanism significantly reduces the risk of passive eavesdropping or active tampering during email transit to your domain.

A key benefit of this inbound protection is that it MTA-STS protects against downgrade attacks, where attackers might try to force an insecure connection. For more details on inbound protection, refer to the Microsoft documentation.

Outbound mail protection with MTA-STS

While publishing an MTA-STS policy protects your inbound mail, implementing MTA-STS checking on your outbound mail server ensures that emails you send are also delivered securely. When your mail server (as the sender) needs to send an email to another domain, it can check if that recipient domain has an active MTA-STS policy. If such a policy is found, your server will enforce secure TLS connections, much like how external servers treat your inbound mail.

This means that if the recipient domain requires TLS via its MTA-STS policy, your mail server will only send the email if a valid, trusted TLS connection can be established. This protects your outbound communications from interception and tampering, even when sending to domains outside of your direct control. By honoring the MTA-STS policies of recipient domains, you contribute to a more secure email ecosystem overall.

Many email platforms and services allow configuration to use MTA-STS on outbound connections, bolstering the security of your transmitted data. This dual application confirms that MTA-STS secures the entire email path, not just specific segments, making it a powerful tool in your email security arsenal.

The MTA-STS policy and its components

The implementation of MTA-STS involves two primary components: a DNS TXT record and a policy file. The DNS TXT record, specific to the purpose of the MTA-STS TXT record, signals to other mail servers that your domain has an MTA-STS policy. This record includes a version identifier and indicates that a policy exists at a specific HTTPS endpoint.

Example MTA-STS policy file

The policy file itself is a plain text file hosted on a web server at a specific URL. This file outlines the rules for secure email delivery to your domain.

MTA-STS Policy File (example.com)plain

version: STSv1
mode: enforce
mx: mail.example.com
mx: *.example.com
max_age: 86400

The policy file contains important directives like 'mode' (enforce, testing, or none), 'max_age' (how long the policy should be cached), and the 'mx' field. The 'mx' field in an MTA-STS policy specifies the allowed mail servers, ensuring only trusted servers can receive mail for your domain. Senders retrieve this policy via HTTPS, typically using MTA-STS policy fetching on a specific port, to validate the email route.

Mode	Description	Impact
None	No STS validation enforced.	Email is sent as if no policy exists.
Testing	STS policy is announced but not enforced.	Sends reports to help monitor compliance without blocking email.
Enforce	STS policy is strictly enforced.	Email fails if TLS security requirements are not met.

Properly setting up your policy is vital. Knowing the filename for an MTA-STS policy (always `mta-sts.txt`) and understanding the policy modes are foundational for effective deployment.

Enhancing email deliverability with MTA-STS

The combined effect of MTA-STS on both inbound and outbound mail flows is a significant boost to overall email security and deliverability. By ensuring that email is always transmitted over encrypted and authenticated channels, it builds trust between mail servers. This trust can indirectly lead to better inbox placement, as receiving servers are more likely to deliver mail from domains that demonstrate a strong commitment to security.

Stylized email icon moving securely between two server towers.

MTA-STS works in conjunction with other email authentication protocols like SPF, DKIM, and DMARC to form a comprehensive defense against phishing, spoofing, and other email-based attacks. While MTA-STS ensures the transport layer security, DMARC, SPF, and DKIM verify sender identity. Understanding a simple guide to DMARC, SPF, and DKIM is crucial for a holistic security posture. We offer robust DMARC monitoring and reporting that provides insights into your email authentication status, including MTA-STS related flows.

With our platform, you receive

AI-powered recommendations to fix issues and strengthen your policy. Our unified platform brings together DMARC, SPF, and DKIM monitoring with blocklist and deliverability insights, making DMARC accessible to everyone, from SMBs to large enterprises, as well as MSPs.

Final thoughts on MTA-STS

MTA-STS is a vital component of modern email security, providing protection for both inbound and outbound mail. By publishing an MTA-STS policy, you ensure that external mail servers sending to your domain use secure, authenticated TLS connections. Simultaneously, by configuring your own mail servers to respect the MTA-STS policies of recipient domains, you guarantee that your outbound emails are also transmitted securely.

This dual-sided approach significantly mitigates the risk of man-in-the-middle attacks and data interception, bolstering the overall integrity and privacy of your email communications. Implementing MTA-STS is a clear signal of your commitment to secure email practices, which can positively impact your domain's reputation and deliverability.

For organizations serious about email security and ensuring their messages reach their intended recipients securely, understanding and implementing MTA-STS for both inbound and outbound mail flows is an essential step. It's a proactive measure that complements other authentication standards to create a robust and trustworthy email environment.