MTA-STS/TLS-RPT Tools

TLS encryption in SMTP is traditionally "opportunistic", meaning that if a secure channel cannot be negotiated, the email is still sent unencrypted (plain text). MTA-STS was introduced to eliminate this opportunistic encryption, ensuring that emails are encrypted before delivery. Whenever TLS encryption failures occur, the sending server generates a TLS-RPT report and sends it to the domain owner. These reports are sent in JSON format to the email address specified in the domain owner's TLS-RPT DNS TXT record.

Domain owners need TLS-RPT to stay informed about email delivery problems due to TLS encryption failures. It provides crucial information for: Email security: Highlights the risks of unencrypted communication (interception, man-in-the-middle attacks). Compliance: Helps organizations meet regulatory requirements for data protection. Deliverability: Improves email deliverability by identifying and resolving encryption issues.

Configuration involves enabling TLS reports for your domain by creating a TXT record for TLS-RPT and publishing it in your DNS. This record must be published in the subdomain smtp.tls.yourdomain.com. The steps are: 1. Generate a TLS-RPT record: You can use a record generator tool. 2. Enter the notification email address: This is where SMTP TLS reports will be received. 3. Publish the TLS record in your DNS: Create a new TXT record. The record syntax is usually: v=TLSRPTv1; rua=mailto:your-email@your-domain.com. v=TLSRPTv1 specifies the protocol version. rua (Reporting URI(s) for Aggregate Data) specifies where the recipient's mail server should send aggregate reports. Multiple destinations can be configured by separating entries with commas, using mailto: for email addresses or https: for endpoint URLs.

TLS Handshakes can fail for various reasons: Certificate errors: Expired, not yet valid, revoked, unsigned, or unsupported certificates. The solution is usually to contact the certificate provider. Version mismatch: If the TLS versions supported by the sending and receiving MTAs do not match. STARTTLS failures: If the STARTTLS command is not properly implemented. MTA-STS enforcement: If the recipient's MTA-STS policy mode is "enforce", a failed TLS handshake may reject the message. Hostname and identity mismatch: The hostname in the server certificate does not match the hostname of the server being connected to. You should verify that the MX records in the MTA-STS policy file match the domain's MX record. MTA-STS policy issues: mta_sts_policy_not_found: No MTA-STS policy found for the recipient domain. mta_sts_policy_invalid: The found MTA-STS policy is invalid or contains errors. mta_sts_policy_fetch_error: Error trying to retrieve the MTA-STS policy. mta_sts_connection_failure: Secure connection fails due to untrusted certificates or incompatible ciphers. mta_sts_invalid_hostname: The recipient mail server's hostname in the MTA-STS policy does not match the real hostname.

With MTA-STS: TLS-RPT and MTA-STS are complementary protocols. MTA-STS ensures the use of TLS for delivery and rejects emails if encryption fails. TLS-RPT allows domain owners to monitor TLS failures and troubleshoot problems. Together, they prevent message interception in transit and minimize deliverability issues. With DANE: DANE (DNS-based Authentication of Named Entities) allows specifying TLS certificates verified through DNSSEC to prevent man-in-the-middle attacks. While TLS-RPT monitors TLS failures, DANE ensures that only trusted certificates are used. DANE actively prevents TLS downgrade attacks and certificate spoofing, maintaining email integrity. A user mentioned that in Postfix, when a server has an MTA-STS record available, it will override DANE and use MTA-STS exclusively, even if TLSA (DANE) records are available.

Regularly monitor TLS reports to ensure that undelivered messages are not lost, either manually or with a report analysis tool. Ensure that the MTA-STS policy is correctly configured and without syntax errors. Address encryption failures detected in TLS reports without delay. Use only the most recent and compatible versions of the TLS protocol.