How do I install a trusted root certificate on my Mac?

I know just what you mean because I also use an SSL connection to securely access my mail server, keeping things quite a bit more secure on an open wireless wifi network. For a long while I was also getting the error I bet you get:

It turns out that Microsoft has some pretty decent instructions on how to install the certificate that you've gotten from your ISP here: install a trusted cert, but I'll summarize here too!

You first step is to save the certificate to the desktop and double-click on it. It'll launch the Keychain Access application and show you the following dialog box:

Now you'll want to click on the "Keychain" pop-up menu and select "X509 Anchors":

Now click "OK" and enter your administrative password when it prompts you:

That should do the trick. You can verify your certificate's installed properly by going to Applications --> Microsoft Office 2004 --> Office and launching "Microsoft Cert Manager". Choose "Apple Trusted Root Certificate Authorities" and then find your new certificate. Double-click it and you should see what I see here:

That all there is to it. Quit and restart Entourage and you should be clear of that pesky error message forever, or at least until the certificate expires!