You applied a new certificate to you mail server. Now you want to know, if the right certificate with the matching chain is used:
openssl s_client -starttls smtp -showcerts -connect mail.example.com:25 -servername mail.example.com
Now you get the certificate chain and can check if everything is ok.
You should get return code 250 at the end of the test.
Once you see this code, you can leave with