It’s worth noting that OWA only supports CAC/PIV S/MIME email signing, encryption and decryption on Windows platforms.
There are some implications to that for OS X and IOS desktop or mobile users.
On Mac OS X, leveraging a commercial or open source CAC/PIV enablement, the answer is to use an email client such as Apple Mail, Microsoft's Entourage or Outlook, provided the back-end data center or cloud-based Exchange Server allows it.
On iPad/iPhone iOS, Thursby hit the same situation, our PKard Reader secure browser app supports secure email with OWA but we had to introduce the PKard Mail app licensing Microsoft's Exchange ActiveSync (EAS) technology in order to enable S/MIME CAC/PIV email signing and encryption.
EAS has to be enabled by the back-end data center or cloud-based Exchange Server.