Configure Exchange certificates

I have set up Microsoft Exchange for multiple domains. E-mail works fine using Outlook Web Access ([login to view URL]). However I can't get certificates to work properly. The server should be hosting some twenty e-mail domains ( john_doe (a) [login to view URL], jane-smith (a) [login to view URL], etc.) and I don't want to buy more certificates that I need. If I understood correctly, a certificate should be only needed for [login to view URL] and autodiscover.myserver.net. I would like to employ an Exchange admin to first instruct me on what certificates I need (preferably from [login to view URL]), then creating the certificate signing request and finally installing and configuring that to the Exchange server. Email functionality should be validated using 1) OWA, 2) Mozilla thunderbird and 3) MS Outlook. I am more than happy to help with the work.