What is the certificate message that you're getting?
self-signed certificates are one problem, expired certificates are another, and certificates whose names don't match the hostname you're accessing are yet a third...
Other than that, you should have the option when looking at the Outlook certificate error to "install" it on your computer. If not, try browsing to webmail on your SME server using Internet Explorer, and install the certificate from there.
The 'resolution' section of this Microsoft KB describes how to 'install' the certificate from within Outlook:
http://support.microsoft.com/kb/2006728