Do you wanna verify the configuration for your certificate in your SMTP server? It’s as easy as this:

openssl s_client -connect smtpin.csic.es:25 -starttls smtp -CApath /etc/ssl/certs

Substitute ‘smtpin.csic.es’ with your server.

The route for your root certificates (/etc/ssl/certs), which is necessary for verifying them, might vary depending on your linux distro. That route should be fine for debian and ubuntu based distros.