MTA-STS (Mail Transfer Agent Strict Transport Security)
Summary
MTA-STS forces TLS encryption for email transmission between mail servers. It prevents downgrade attacks and man-in-the-middle interception by requiring encrypted connections through policy files published via HTTPS and DNS.
What Is MTA-STS (Mail Transfer Agent Strict Transport Security)?
MTA-STS (RFC 8461) ensures email is transmitted over encrypted TLS connections between mail servers. Unlike opportunistic TLS, MTA-STS enforces encryption through a policy file at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt.
Why MTA-STS (Mail Transfer Agent Strict Transport Security) Matters for Email & DNS Security
Without MTA-STS, email can be downgraded to unencrypted transmission through STARTTLS stripping attacks. Attackers can read sensitive emails, modify messages, or harvest credentials. According to IntoDNS data, MTA-STS adoption remains low at approximately 8% of domains, despite its critical role in preventing man-in-the-middle attacks. MTA-STS is critical for compliance (GDPR, HIPAA).
How MTA-STS (Mail Transfer Agent Strict Transport Security) Works (Technical)
- 1.Sending server queries DNS for _mta-sts.yourdomain.com
- 2.DNS record contains v=STSv1; id=version
- 3.Fetches policy from https://mta-sts.yourdomain.com/.well-known/mta-sts.txt
- 4.Policy specifies mode (enforce/testing), mx servers, max_age
- 5.Validates MX hostname matches policy
- 6.Enforces TLS or fails delivery
- 7.Policy cached for max_age duration
Common Misconfigurations
❌ mode: testing instead of enforce
Consequence: No protection - monitoring only
How IntoDNS detects this: IntoDNS recommends enforce for production
❌ Policy not served over HTTPS
Consequence: MTA-STS validation fails
How IntoDNS detects this: IntoDNS validates HTTPS accessibility
❌ MX hostname mismatch
Consequence: Legitimate mail rejected
How IntoDNS detects this: IntoDNS compares MX with policy mx: entries
How IntoDNS.ai Detects & Scores This
IntoDNS validates MTA-STS DNS record, HTTPS policy file accessibility, policy mode strength, MX hostname alignment, max_age configuration, and certificate validity. MTA-STS accounts for 10% of email security score.
How To Fix MTA-STS (Mail Transfer Agent Strict Transport Security) Issues
- 1.Create policy: mode: enforce, mx: mail.domain.com, max_age: 86400
- 2.Host at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt
- 3.Ensure valid HTTPS certificate
- 4.Create DNS TXT: _mta-sts.yourdomain.com → v=STSv1; id=20240101
- 5.Start with mode: testing, monitor 2 weeks
- 6.Update to mode: enforce
- 7.Set max_age to 1 year after stable
References
Source: IntoDNS.ai – DNS & email security diagnostics
Last updated: 2026-01-14
Category: security