Digitally Signed Email Can Not Be Opened in Outlook 2003 Client

I have an end user who received a digitally signed email, of course it does not show in the preview pane but when he attempts to open it, it generates the following error message:

"A Required Action was not Successful due to an Unspecified Error"

The part that I find strange is that the email and attachments can be opened and viewed through Outlook Web Access but not with the Outlook client on his workstation. Can anyone offer some insight as to why this might be the case? He is running Outlook 2003 SP3.