Problem on Email with Digital Certificate using Outlook 2010

I have upgraded Windows XP Service Pack 3 workstation to Outlook 2010 from Outlook 2000. The problem is when I am receiving email with digital certificate, the contents of the email cannot be viewed and the error message displays that there is no digital certificate. The sender of the mail is already in the contact list and it was working properly in Outlook 2000. When I checked, I noticed that sender does not have a digital certificate in the contact list of Outlook 2010. I’m not sure but maybe this is the cause of the problem. Please help me.
