From Roaring Penguin
If you like, you can obtain this information from a UNIX/Linux machine by running the command:
- openssl s_client -CApath path_to_certificate -showcerts -starttls smtp -connect example.com:25
- example.com is the host name of the server being checked.
- path_to_certificate is the path to where the certificate is stored on the machine being queried, eg. on Debian it's typically /etc/ssl/certs