• DNS TXT Record: Published at _mta-sts.yourdomain.com containing the policy ID version (e.g., v=STSv1; id=20240101T000000;). The ID must change when the policy is updated.
• Policy File: Hosted at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt containing the policy mode, MX patterns, and max age. Must be served over HTTPS with a valid certificate.
• Policy Modes: Three modes available - "enforce" (strict validation required), "testing" (for monitoring without enforcement), and "none" (policy disabled).
• MX Patterns: Wildcard patterns specifying which mail servers are authorized to receive email for the domain (e.g., mx: *.example.com).
• Max Age: Specifies how long (in seconds) the policy should be cached by sending mail servers. Recommended value is 86400 (1 day) or higher.

Why Implement MTA-STS?

MTA-STS protects email in transit by ensuring that connections between mail servers are encrypted and authenticated. It prevents attackers from intercepting or downgrading connections to unencrypted channels, which is critical for protecting sensitive email communications.

Implementation Steps

1. Create a policy file with your configuration (mode, mx patterns, max_age)
2. Host the policy file at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt with a valid HTTPS certificate
3. Publish a TXT record at _mta-sts.yourdomain.com with the policy version ID
4. Start with "testing" mode to monitor without enforcement, then transition to "enforce" mode once verified