Why is the security classification drop down menu not available when I compose an email in Outlook?

The security classification drop down menu will not be available if Outlook has been set to use Microsoft Office Word to edit email messages.
This feature can be turned off in Outlook by going: Tools > Options... and choose the Mail Format tab. In this dialog, deselect both Use Microsoft Word to edit e-mail messages and, Use Microsoft Word to read Rich Text e-mail messages.