$ openssl s_client -tls1_2 -cipher ECDHE-RSA-AES128-GCM-SHA256 -starttls smtp -verify 3 -verify_return_error -debug -CApath /etc/ssl/certs -connect 188.8.131.52:25
"-tls1_2" forces the TLSv1.2 protocol. Make sure protocol and cipher list match.
"-verify 3" enables server certificate verification and sets the length of the certificate chain. In this case there are 3 certificates in the certificate chain, including the root CA. Make sure the public root CA certificate is in the "-CApath" directory. "-verify_return_error" enforces the certificate verification to succeed.
The "-cipher" option specifies the list of ciphers to be transferred to the server. The server then decides which of these ciphers to use. As we only give one cipher, we force the postfix server to only use this one. If the server does not support this cipher, openssl will return with an error.
If everything goes well, you will see a long output from the server (including the protocol and cipher from your openssl command line options) and something like "Verify return code: 0 (ok)". Quit the connection with the postfix server by typing "quit" and hit return.