This article tells it pretty clearly:

http://www.cyberciti.biz/tips/postfix-smtp-ssl-certificate-csr-installat...

Here are steps to install the digital SSL certificate on Qmail server:

Here are some references for educational purposes.

Here are some references on credential revocation:

To import a new CA certificate into Thunderbird 1.5:

1. Launch Thunderbird and choose Account Settings from the Edit menu.
2. Select Security under the account name.
3. Click the View Certificates button.
4. Click the Authorities tab.
5. Click the Import button at the bottom of the screen.
6. Navigate to the CA certificate and import it.

To import a new CA certificate into Evolution:

1. Launch Evolution and choose Preferences from the Edit menu.
2. Select Certificates
   

Browsers ships with certificates from well-known commercial CAs. If your web servers use certificates signed by your own CA, your CA certificate must be exported to every browser on every client system that will be connecting to an SSL-enabled web server.