What is MTA-STS?
MTA-STS (Mail Transfer Agent Strict Transport Security) is a security standard that enables email domains to require encrypted (TLS) connections for incoming mail, preventing downgrade attacks.
MTA-STS (Mail Transfer Agent Strict Transport Security) allows domain owners to declare that incoming email must use TLS encryption. Without MTA-STS, sending servers might fall back to unencrypted transmission if they encounter TLS errors—a vulnerability that attackers can exploit.
How MTA-STS works: 1. Domain owner publishes an MTA-STS policy file at https://mta-sts.example.com/.well-known/mta-sts.txt 2. Domain owner adds a DNS TXT record pointing to the policy 3. Sending servers check for MTA-STS before delivering mail 4. If MTA-STS is present, senders MUST use TLS or reject the delivery 5. Downgrade attacks become impossible—no fallback to unencrypted
MTA-STS prevents man-in-the-middle attacks where attackers strip TLS from email connections, allowing them to read message contents.
Why MTA-STS Matters
Email encryption in transit has historically been opportunistic—TLS is used when available but silently dropped otherwise. Attackers can exploit this by forcing connections to downgrade to unencrypted. MTA-STS prevents these downgrade attacks, ensuring your email remains encrypted throughout delivery. This is especially important for sensitive transactional emails.
How Ark Handles MTA-STS
Ark's sending infrastructure always attempts TLS and respects MTA-STS policies published by receiving domains. When you configure your own domain with Ark, we recommend implementing MTA-STS to protect incoming mail to your organization. Our documentation covers MTA-STS setup alongside other authentication standards.
Frequently Asked Questions
How is MTA-STS different from STARTTLS?
STARTTLS upgrades an unencrypted connection to TLS, but it can be stripped by an attacker. MTA-STS tells senders 'you must use TLS or don't deliver at all'—removing the downgrade option.
Do I need MTA-STS if I have TLS certificates?
TLS certificates enable encryption, but without MTA-STS, sending servers might accept self-signed certs or fall back to unencrypted. MTA-STS enforces that only valid TLS connections are accepted.
What happens if MTA-STS causes delivery failures?
Start with mode=testing to monitor without enforcing. This sends reports about what would have failed. Once confident, switch to mode=enforce. Properly configured senders will always succeed; misconfigured ones will fail.
How does MTA-STS relate to DANE?
Both prevent TLS downgrade attacks but use different mechanisms. DANE uses DNSSEC, which has lower adoption. MTA-STS uses HTTPS and standard DNS, making it easier to deploy. They can coexist for defense in depth.
Related Terms
Email Authentication
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DKIM
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DMARC
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is an email authentication p...
SPF
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