MTA-STS
Checker

What is MTA-STS and Why It Matters

MTA-STS (RFC 8461) is a security standard that enforces Transport Layer Security (TLS) for email exchanges between mail servers. It prevents downgrade attacks where attackers force connections to use unencrypted or weakly encrypted channels. By publishing an MTA-STS policy, you ensure that legitimate mail servers always use encrypted connections when delivering mail to your domain, protecting against eavesdropping and tampering.

MTA-STS vs DANE

While DANE (DNS-based Authentication of Named Entities) provides similar protection, it requires DNSSEC which has limited adoption. MTA-STS works without DNSSEC by using HTTPS to deliver the policy file, making it more widely deployable. Major email providers like Google, Microsoft, and Yahoo support MTA-STS, making it an essential security layer for modern email infrastructure.

MTA-STS Deployment Steps

1. Create and host the policy file at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt
2. Start with mode: testing to monitor without enforcement
3. Add DNS TXT record at _mta-sts.yourdomain.com with unique ID
4. Implement TLS-RPT to receive failure reports
5. Monitor reports for 2-4 weeks to identify issues
6. Switch to mode: enforce when confident
7. Update ID in DNS record when policy changes

Common MTA-STS Issues

The most common MTA-STS failures stem from incorrect MX server listings in the policy file. All legitimate mail servers must be listed with their exact hostnames. Certificate mismatches occur when the mail server's TLS certificate doesn't match the hostname. The policy file must be served over HTTPS with a valid certificate. DNS caching can delay policy updates, so plan changes carefully. Always test in testing mode before enforcement to avoid blocking legitimate mail.