How can I use my secure certificate for my email server?

This will entirely depend on your operating system, and mail server. Some general guidlines are listed here:

Your cert will usually need to be in 'pem' fashion. That is, both key and cert in one file. Your mail server must support SSL/TLS. For example, if you have a CPanel server, you can use your cert by editing the following files to contain your cert/key:

/etc/exim.key

/etc/exim.crt

/usr/local/cpanel/etc/cpanel.pem

After your certificate has been installed, you will need to restart CPanel, which should automatically restart Exim.