The short answer is no. MTA-STS (Mail Transfer Agent Strict Transport Security) does not replace the need for DMARC. They are both essential email security standards, but they solve completely different problems. Think of it like this: DMARC verifies who sent an email, while MTA-STS secures how that email gets from one server to another.
Using one without the other leaves significant security gaps. An email could be perfectly authenticated by DMARC but still get intercepted and read in transit if the connection isn't secure. Conversely, an email could travel over a securely encrypted channel but still be a phishing attempt from a spoofed domain. For complete email security, you need both.
What is DMARC?
DMARC stands for Domain-based Message Authentication, Reporting, and Conformance. Its primary job is to protect your domain from being used in phishing and spoofing attacks. It acts as a policy layer on top of two other authentication standards: SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail).
In simple terms, DMARC allows a domain owner to tell receiving email servers what to do with messages that fail SPF or DKIM checks, meaning they can't be verified as legitimate. You can instruct receivers to monitor the emails (p=none), send them to spam (p=quarantine), or block them completely (p=reject). This is all about authenticating the sender's identity.
What is MTA-STS?
MTA-STS is a newer standard designed to secure the connection for email in transit. Most email servers today use something called opportunistic TLS, which means they will try to establish an encrypted connection using a command called STARTTLS. However, if an encrypted connection can't be established, they will often fall back to sending the email over an unencrypted, plaintext connection. This makes them vulnerable to man-in-the-middle (MITM) and downgrade attacks, where an attacker can force the connection to be unencrypted and then read or modify the email's contents.
MTA-STS solves this by allowing domains to publish a policy that requires sending servers to use a secure, encrypted TLS connection. If a secure connection cannot be established, the email will not be delivered. It enforces encryption for the email's journey between servers.
How do DMARC and MTA-STS work together?
They address two distinct, but equally important, parts of email security: authentication and encryption. As noted in a comprehensive guide, the protocols that DMARC builds upon are focused on verifying authenticity, while MTA-STS is all about the secure transport.
Here is a breakdown of their different functions:
- DMARC verifies the 'who'. It confirms that the email is from a legitimate, authorized sender and hasn't been forged. It protects your brand and your recipients from phishing.
- MTA-STS secures the 'how'. It ensures the path the email takes between mail servers is private and encrypted, protecting the content of the email from being intercepted or altered.
- They operate at different stages. DMARC checks are performed by the receiving server once the email arrives to validate the source. MTA-STS policies are checked by the sending server before it even transmits the email to ensure a secure channel is available.
In conclusion, MTA-STS is not a replacement for DMARC. A modern, comprehensive email security strategy requires implementing both. DMARC protects your domain's identity, and MTA-STS protects the data in your emails as it moves across the internet. Together, they provide layered security that addresses the most common and dangerous email-based threats.
