You can use OpenSSL. If you have to check the certificate with STARTTLS, then just do openssl s_client -connect mail.example.com:25 -starttls smtp or for a standard secure smtp port: openssl s_client -connect mail.example.com:465
You should use openssl s_client, and the option you are looking for is -tls1_2. An example command would be: openssl s_client -connect google.com:443 -tls1_2 If you get the certificate chain and the handshake you know the system in question supports TLS 1.2. If you see don’t see the certificate chain, and something similar to “handshake … Read more