[Full-Disclosure] SMTP Encryption (S/MIME) for Outlook question

No flames here please.  I've just been asked about running some form of
encryption on our mail clients (Outlook) to send encrypted SMTP across the
Internet and would like some opinions/directions.

Our userbase isn't that technical so we'd need something that is pretty user
friendly (I know, divergent goals) but is still secure to a point.

I don't know the exact details on their goals other than preventing random
eavesdropping (sniffing) of clear-text SMTP traffic across the Internet to a
remote, non-internal destination.

What do other Win/Exchange/Outlook IT admins use for S/MIME?  

BTW, if there's something that will run on top of the SMTP gateway server or
the internal Exchange server to encrypt the message before being routed to
the Internet, this is also acceptable.  I figure there must be something
available that works like this.

TIA,

Brandon Fetch
817-871-4036
-- carpe ductum -- "Grab the tape"