Click Contacts in the Folder List.
You can also open the Address Book.
Double-click a contact, then click the Certificate tab.
Click a certificate, then click View Details.
If you initially did not trust a recipient's security certificate and want to trust it, open a digitally signed item from the recipient, click the security certificate, click Modify Trust, click a trust option, then click OK.
If you no longer want to trust a recipient's security certificate, click the security certificate, click Remove, then click Yes.
When you remove a recipient's security certificate from the list, it is removed from your certificate database. If you receive an item using that security certificate in the future, it will be considered unknown.