Vanguard wrote:

So I suspect that you could leave encryption always enabled but it
won't encrypt except for those recipients that have previously sent
you their public key in a digitally signed mail (that you need to
save in your contact folder).

I'll confirm this. Some of the people in the company for which I work
exchange encrypted mail with people in another company. They have
encryption enabled all the time and if there's a cert for the recipient, the
message will be encrypted. If there is no cert, then there is no
encryption. Outlook handles it automatically.

As for the person being in the Contacts folder, it's not necessary in the
case of a publically -available LDAP server in which Outlook can perform a
query for the recipient. That's how we have things set up. We reference an
LDAP server in which can be found the names of those people who have certs.
Outlook will first look in the Contacts folder and then query the LDAP
server if the name isn't in the folder.
--
Brian Tillman