Hello, I have a user who just switched from Mac Mail to Outlook 2011. Getting a strange issue when sending to users on Outlook 2010 PC. When sending a message, users on Outlook 2011 and people viewing the message via web mail can see the message fine. It reports that it's a digitally signed message (just one sentence), and the certificates look fine. Via the web, the message appears fine. It says: "This message has a digital signature. The digital signature could not be validated because the S/MIME control is not available." However, it comes up fine. However, users on Outlook for PC (my version is 2010), cannot read it. I first get a preview pane error "This item cannot be displayed in the Reading Pane. Open the item to read its contents." And when I double-click on the message, I get a pop up window "Your Digital ID name cannot be found by the underlying security system." Anyone can provide clues why other Outlook MAC users and via the web the message can be read, but just not PC Outlook users for now? Thanks,.