Según recuerdo no es posible verlo sino hasta Exchange server 2013 SP1..
mira lo que dice el articulo:
https://technet.microsoft.com/en-us/library/jj907309(v=exchg.150).aspx
Exchange 2013 SP1 now supports S/MIME-based message security with Outlook Web App.
Secure/Multipurpose Internet Mail Extensions (S/MIME) allows people to help protect sensitive information by sending signed and encrypted email within their organization.
Administrators can enable S/MIME for mailboxes by synchronizing user certificates and then configuring Outlook Web App to support S/MIME.
For more information, see S/MIME for message signing and encryption and the Get-SmimeConfig cmdlet reference.
Puedes hacer la prueba de no enviar el correo... guardar el correo como Draft y ve si se observa ese adjunto,
muy probablemente ahí si lo veas.
Saludos!
