Why does Postfix show TLS handshake failure when connecting to an Exchange server?
Summary
What email marketers say7Marketer opinions
Email marketer from ServerFault responds that the issue is usually either that your client doesn't trust the certificate presented by the server, or that the cipher suites don't overlap. You can verify the certificate is valid. Also you can force the the client to use less secure settings that overlap more with the server.
Email marketer from Reddit notes that common TLS problems include expired certificates, incorrect DNS configuration preventing certificate validation, firewall blocking TLS ports (465, 587, 993, 995), and SNI (Server Name Indication) issues. Ensure the Exchange server's certificate is valid, DNS is properly configured, firewalls are allowing TLS traffic, and that SNI is correctly configured if required.
Email marketer from StackOverflow suggests using `openssl s_client -starttls smtp -connect your.exchangeserver.com:25` to diagnose the TLS connection. This command can help identify certificate issues, protocol negotiation problems, and cipher suite mismatches by providing verbose output of the TLS handshake process.
Marketer from Email Geeks shares an SSL Labs test link and mentions a possible problem with the TLS chain after the assessment failed with "No secure protocols supported".
Email marketer from ExpertsExchange responds that problems with Certificate Revocation List (CRL) can sometimes cause handshake issues. Ensure the Postfix server can access the CRL distribution points specified in the Exchange server's certificate. If CRL access is blocked (e.g., by a firewall), the TLS handshake may fail.
Email marketer from EmailSecurity suggests that using weak or outdated encryption protocols or cipher suites can lead to TLS handshake failures. Enable strong ciphers and protocols. Remove any weak or outdated configurations from both Postfix and Exchange.
Email marketer from LinuxQuestions discusses that Maximum Transmission Unit (MTU) size issues can sometimes cause TLS handshake failures. Check the MTU size on the Postfix server's network interface and ensure it is compatible with the network path to the Exchange server. If necessary, adjust the MTU size to avoid fragmentation.
What the experts say1Expert opinions
Expert from Word to the Wise recommends using `openssl s_client` to test TLS connections and diagnose issues. She notes this can help pinpoint if the problem is with certificate validation, protocol negotiation, or cipher suite compatibility. This method is useful for troubleshooting TLS handshake failures between Postfix and Exchange servers.
What the documentation says4Technical articles
Documentation from Microsoft Learn explains that TLS handshake failures can occur due to certificate issues (expired, revoked, or untrusted), protocol mismatches (server and client not supporting a common protocol), cipher suite incompatibility, or network issues interrupting the handshake process. Ensure both the Postfix and Exchange server have valid certificates, support compatible TLS versions and cipher suites, and that no firewalls are blocking the necessary ports.
Documentation from Microsoft Learn discusses ensuring that Exchange Server is configured to support TLS 1.2 or higher. TLS 1.0 and 1.1 are deprecated and may cause handshake failures. Update Exchange server to use modern TLS protocols and cipher suites.
Documentation from Postfix.org details the various `smtp_tls_*` and `smtpd_tls_*` configuration parameters. Check settings like `smtp_tls_security_level`, `smtp_tls_mandatory_protocols`, `smtp_tls_mandatory_ciphers`, `smtpd_tls_cert_file`, and `smtpd_tls_key_file`. Ensure the certificate and key files are correctly configured and accessible, and that the TLS settings are compatible with the Exchange server's requirements.
Documentation from OpenSSL.org recommends checking the OpenSSL configuration files for any restrictions on TLS versions or cipher suites. Also check that the Exchange server's certificate is trusted.