What Is MTA-STS?

What Is MTA-STS?

MTA-STS (Mail Transfer Agent Strict Transport Security) is a security standard that enforces TLS encryption for SMTP connections, preventing downgrade attacks and man-in-the-middle attacks. MTA-STS enables mail servers to require TLS encryption for email delivery, ensuring emails are transmitted securely.

MTA-STS works by: publishing MTA-STS policy in DNS and HTTPS, mail servers checking MTA-STS policy before connecting, enforcing TLS encryption for SMTP connections, and preventing downgrade attacks that force unencrypted connections.

MTA-STS enhances email security by ensuring encrypted email transmission and protecting against SMTP security vulnerabilities. Learn more about STARTTLS and SMTP security.

MTA-STS Purpose

1. TLS Enforcement

MTA-STS enforces TLS encryption for SMTP connections, ensuring emails are transmitted securely over encrypted connections.

2. Downgrade Attack Prevention

MTA-STS prevents downgrade attacks that force unencrypted SMTP connections, protecting against man-in-the-middle attacks.

3. SMTP Security

MTA-STS enhances SMTP security by requiring encrypted connections and preventing security vulnerabilities.

4. Email Privacy

MTA-STS protects email privacy by ensuring email content is encrypted during transmission.

5. Compliance

MTA-STS helps organizations comply with email security requirements and best practices.

How MTA-STS Works

1. Policy Publication

Domain owners publish MTA-STS policy in DNS (_mta-sts.example.com) and serve policy file over HTTPS (https://mta-sts.example.com/.well-known/mta-sts.txt).

2. Policy Discovery

Sending mail servers discover MTA-STS policy by querying DNS and retrieving policy file from HTTPS endpoint.

3. Policy Evaluation

Sending mail servers evaluate MTA-STS policy to determine if TLS encryption is required for SMTP connections.

4. TLS Enforcement

If MTA-STS policy requires TLS, sending mail servers enforce TLS encryption and reject unencrypted connections.

5. Connection Security

SMTP connections are established over TLS, ensuring encrypted email transmission and protection against attacks.

MTA-STS Requirements

1. DNS Record

MTA-STS DNS record must be published: _mta-sts.example.com TXT "v=STSv1; id=policy-id;"

2. HTTPS Policy File

MTA-STS policy file must be served over HTTPS: https://mta-sts.example.com/.well-known/mta-sts.txt

3. TLS Support

Mail servers must support TLS encryption (STARTTLS) for MTA-STS to work.

4. Policy Format

MTA-STS policy file must follow specific format with version, mode, max_age, and mx records.

5. Certificate Validation

MTA-STS requires valid TLS certificates for HTTPS policy file and SMTP connections.

Implementing MTA-STS

1. Enable TLS on Mail Servers

Ensure mail servers support TLS encryption (STARTTLS) for SMTP connections.

2. Create MTA-STS Policy File

Create MTA-STS policy file with version, mode (enforce, testing, none), max_age, and mx records.

3. Serve Policy File Over HTTPS

Serve MTA-STS policy file at: https://mta-sts.example.com/.well-known/mta-sts.txt

4. Publish DNS Record

Publish MTA-STS DNS record: _mta-sts.example.com TXT "v=STSv1; id=policy-id;"

5. Start with Testing Mode

Start with mode: testing to monitor MTA-STS without enforcing, then progress to mode: enforce.

6. Monitor and Verify

Monitor MTA-STS implementation to ensure TLS connections are working correctly and emails are being delivered securely.