Publish S/MIME certificates for external contacts to Active Directory for use with Exchange Server 2007
In the past, if you wished to use S/MIME for e-mail encryption with an external recipient, you would add the recipient to your Contacts folder. This was typically done by having the recipient send you a digitally signed item and then right click on the recipient in the From field and click “Save to Contacts”.
Read more at source: http://msexchangeteam.com/archive/2008/04/23/448761.aspx
