MTA-STS Implementation Guide
Secure your email transport with MTA-STS
Learn how to implement and manage MTA-STS (Mail Transfer Agent Strict Transport Security) to protect your email communications from downgrade attacks.
What is MTA-STS?
MTA-STS (Mail Transfer Agent Strict Transport Security) is a security standard that enables mail servers to use secure TLS connections when sending email. It helps prevent downgrade attacks and ensures email is transmitted securely.
Implementation Steps
1. Create TLS-RPT Record
Add a TLS-RPT record to monitor TLS failures:
_smtp._tls.yourdomain.com. IN TXT "v=TLSRPTv1; rua=mailto:tls-reports@yourdomain.com"
This record enables receiving TLS-RPT reports from major email providers.
2. Create MTA-STS Policy File
Create your MTA-STS policy file with testing mode:
version: STSv1 mode: testing max_age: 86400 mx: mail1.yourdomain.com mx: mail2.yourdomain.com
Include all MX records that will be used for sending email.
3. Publish MTA-STS Policy File
Host your policy file at:
https://mta-sts.yourdomain.com/.well-known/mta-sts.txt
Ensure your web server serves the file with the correct headers:
Content-Type: text/plain Cache-Control: max-age=86400
Verify the file is accessible via HTTPS and has the correct MIME type.
Platform-Specific Guides
Need help publishing your MTA-STS policy file? Check out our platform-specific guides:
4. Create Discovery Record
Add an MTA-STS discovery record to your DNS:
_mta-sts.yourdomain.com. IN TXT "v=STSv1; id=20240101000000;"
Note: Update the id value whenever you make changes to your policy.
5. Monitor TLS-RPT Reports
Monitor TLS-RPT reports for 2-4 weeks to ensure:
- All legitimate email flows are working correctly
- No unexpected TLS failures are occurring
- All MX records are properly configured
- SSL certificates are valid and trusted
Address any issues found in the reports before proceeding to enforce mode.
6. Enable Enforce Mode
Once monitoring shows no issues, update your policy to enforce mode:
version: STSv1 mode: enforce max_age: 2592000 mx: mail1.yourdomain.com mx: mail2.yourdomain.com
And update your discovery record with a new ID:
_mta-sts.yourdomain.com. IN TXT "v=STSv1; id=20240201000000;"
The new ID ensures all mail servers will fetch the updated policy.
Implementation Phases
Phase 1: Testing Mode
Start with testing mode to validate your configuration:
version: STSv1 mode: testing max_age: 86400 mx: mail1.yourdomain.com mx: mail2.yourdomain.com
Phase 2: TLS-RPT Report Review
Before moving to enforce mode:
- Monitor TLS-RPT reports for 2-4 weeks
- Review all TLS failures and their reasons
- Address any identified issues:
- Update MX records if needed
- Fix TLS certificate issues
- Resolve any mail server configuration problems
- Ensure all legitimate email flows are working correctly
Phase 3: Enforce Mode
Once TLS-RPT reports show no failures, update your policy to enforce mode:
version: STSv1 mode: enforce max_age: 2592000 mx: mail1.yourdomain.com mx: mail2.yourdomain.com
Best Practices
Begin with testing mode to validate your configuration before enforcing strict policies.
Monitor reports to identify and resolve any TLS-related issues before they impact email delivery.
Increment the policy ID to ensure all mail servers fetch the updated policy immediately.
Longer cache times reduce DNS lookups and improve performance while maintaining security.
Expired or invalid certificates will cause TLS failures and email delivery issues.
Ensure all MX records in your policy match your actual mail server configuration.
Common Issues and Solutions
Policy File Not Accessible
Ensure your policy file is accessible via HTTPS and has the correct MIME type. Check your web server configuration and SSL certificate.
DNS Record Issues
Verify that your DNS TXT record is correctly formatted and propagated. Use DNS lookup tools to confirm the record is visible.
TLS-RPT Report Analysis
- Check MX record configurations
- Verify SSL certificate validity
- Ensure mail servers support TLS 1.2 or higher
- Review any intermediate certificate issues
Test Your Implementation
Use our MTA-STS testing tool to verify your configuration:
Test Your MTA-STS Configuration