This is something I come across every now and then.  If someone has an SSL cert on their OWA/Outlook Anywhere server that issued by someone like GoDaddy, they may not be able to access their account from an older phone.  The notes below are pretty much pulled from GoDaddy’s document on how to setup the phone correclty.  There are similar instructions for other certificate authorities as well, if you’re having problems feel free to send me a message @goodbadtech and I’ll see if I can help out.

GoDaddy ’s Valicert root certificate is installed on all mobile devices that run Windows Mobile 5.0 AKU 2 or a later incarnation of the operating system. However, devices that run older versions of Windows Mobile 5.0 do not have the Go Daddy root installed.

To check if the Go Daddy root is installed on your device, please visit the root store on your device:

• Open the “Settings” menu.
• Select “System.”
• Select “Certificates.”
• Verify that the “http://www.valicert.com” is listed in the root store.
• If the root is included, your device is running Windows Mobile 5 AKU 2 or later. No further action is required.
• If the root is not included, follow the instructions below to import and install it.

To install the root certificate on your Windows Mobile 5 device:

• Download the root certificate to your PC in DER format with a .cer file extension (i.e., valicert_class2_root.cer”). The root can be downloaded from the Go Daddy repository.
• Copy the downloaded root certificate to your device using ActiveSync.
• On your mobile device, locate the imported file using File Explorer and click on it.
• The device will display the following prompt: “You are about to install valicert_class2_root.cer certificate issued by http://www.valicert.com/. Do you want to continue?” (If you saved the root under a different name, that file name will show up in the prompt.)
• Accept the prompt to install the root certificate on your device.

https://certs.godaddy.com/InstallationInstructions_alt.go