There are a few requirements which must be satisfied before you can
send digitally signed or digitally encrypted email via the OWA.
Your Secure Email Certificate
must first be installed on the computer you are using the OWA from, or
it should be published to the Global Address List (using Outlook
2003/2007/2010 via the MAPI Protocol)
Windows 7 / Windows Vista users must run Internet Explorer as
an Administrator (by right clicking on the icon and selecting "Run as
Administrator), or disable the UAC.