Please could you follow the following steps
1. Log in to webmail.
2. In left hand pane, click Options.
3. On right hand pane, scroll down a little bit till you see Security. Under which you can see a download button under S/MIME.
4. Download and install the latest S/MIME.
5. Now close Internet Explorer and open IE again.
6. Log into webmail and try to compose a new mail.