Unable to sign or encrypt email messages

I am running Window XP with Outlook 2003. I have setup Outlook to send
certificates and/or encrypt email messages. But everytime I attempt to send I
receive the following message: Microsoft Outlook could not sign or encrypt
this message because your certificate is not valid.

I know the certs work because the user is currently using cac card with
certs to send certificates on their current Window XP with Outlook 2003
workstation to internal and external addresses. I have removed the certs, put
them back in place, made certs available to windows through the activcard
client, and checked the checked the configuration over and over without any
success.

Is there any idea why I keep receiving this error?