Mac Mail and Network Solutions SSL Certificate

We've just enabled SSL Support on our email server (iMail Server 11).
I'm trying to setup a test Mac Mail account.

I'm getting the following error:
"This certificate was signed by an unknown authority".

This is a new, trusted certificate from Network Solutions.
This same symptom is happening on 4 different Macs.
The certificate is under mailhost.myorderdesk.com

https://mailhost.myorderdesk.com work perfectly in safari and all other Windows broswers.

I've got 5 windows machines running flawlessly on differnt clients (Express, Outlook 07 and Outlook 03)

What am I missing?

Have you installed the intermediate certificate, if any, on your mail server?

I've installed the intermediate and root certificates in Certificate Manager (It's a windows server).

Maybe that's just for IIS?
In the iMail Server SLL Utility I can't find any place to install the Intermediate and Root certificates.