How to inspect the certificate of a mail server over a CLI

If you ever need to inspect the certificate of a remote SMTP server, you can use the openssl CLI tool.

If you need to check STARTTLS:

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

To save the certificate to a file, just redirect the output:

openssl s_client -connect mail.example.com:25 -starttls smtp > mail.example.com.crt

You can also check SMTP TLS using MX Toolbox or Check TLS.