Firstly when your issuance email arrives it will contain your web server certificate. Copy your web server certificate into a text editor such as notepad including the header and footer. You should then have a text file that looks like:
-----BEGIN CERTIFICATE-----
[encoded data]
-----END CERTIFICATE-----
Make sure you have 5 dashes to either side of the BEGIN CERTIFICATE and END CERTIFICATE and that no white space, extra line breaks or additional characters have been inadvertently added.
Stop both Web Messaging and Web Calendaring. In your IMail directory, create a backup copy of the keyname.crt file created when you first ran through the IMail SSL Configuration Utility. Then, open the keyname.crt file in a text-only editor (such as Notepad.exe) and replace the information in the file with the information from your CA. Then, restart Web Messaging/Calendaring. Your CA-signed SSL Certificate should now be active.