Add a sender's certificate to the Address Book
If someone has sent you an e-mail message that is digitally signed with a certificate, you can save the certificate in Outlook. If you have a person's certificate in Outlook, you can send that person an encrypted message.
Open a message that is digitally signed.
In the Info Bar at the top of the message, click Details, and then click Add Encryption Certificate to Contacts.
The certificate is stored with your contact entry for this sender. If you do not already have this person saved as a contact, Outlook automatically creates a contact entry.
Note: A certificate is automatically associated with a particular e-mail address. If you have multiple e-mail addresses for a contact, you can only send an encrypted message to the address that you have a certificate for.