S/MIME capable mail application for OS X

Outlook 2011 is the only application of which I am aware that uses Apple's keychain to store digital certificates for S/MIME.

Have a look at this -- I believe it takes care of what you're asking about, though it's actually an add-on to Mail.app, as opposed to a stand-alone mail client with built-in encryption functionality. It was a dead project about a year ago, and Snow Leopard support wasn't available but some folks picked it up and turned it into a friendly install package. My testing shows it to be pretty stable.

http://www.gpgtools.org/installer/index.html

You can also install individual components from the suite if you want to. Specifically I installed gpgmail (see link below), because I already had a gpg keychain manager in place. But if I were starting from ground zero, I'd probably go with the overall gpgtools installer.

http://www.gpgtools.org/gpgmail/