To select a signing certificate, perform the following steps:
In two-pane layout, From the Service menu, click Options. The Options section appears.
In three-pane layout, From the drop-down list at the top right corner of the Convergence UI, select Settings. The Settings section appears.
If message signing is enabled at your site, the Security option appears in the left panel under the Mail options.
Click Security.
Select the signing certificate you want to use from the Signing Certificate drop-down list.
Click Save Preferences.