As for enterprise solutions: I seriously doubt that PGP is a solution there, because recipients must have a pgp based solution as well. OTOH they can easily run S/MIME and feel fine... own-mailbox, at least, seems to support a mode where the recipients does not need gpg by sending out a link which points back to the plaintext email (and I assume they would then need a password of some kind.)
I've had to use Microsoft products (Windows & Office) in an enterprise environment before and, while I'd much prefer to use Thunderbird & GnuPG, Symantec's commercial PGP products in conjunction with Outlook are a good solution for this environment. A centralized server controls configuration and policies and, once up and running, it just works(TM) and, IME/IMO anyways, was a better solution than Outlook + S/MIME.
Of course, this required both sender and receiver to have/use PGP and/or a compatible product.
