← Glossary
Authentication
What is MTA-STS?
A policy that tells sending servers to deliver mail to your domain only over validated TLS.
How it works in practice
It protects inbound mail against downgrade attacks. The domain publishes a DNS record and a policy file over HTTPS listing its MX hosts and whether to enforce TLS. It protects mail sent to you, not mail you send, so it doesn't affect your outbound deliverability directly.
The common mistake
Expecting MTA-STS to improve inbox placement. It's a security control for receiving mail.
Related terms
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