After you set a registry key to enable this ability for Outlook 2013, digitally signed email messages are displayed in the HTML format when standard email messages are configured to be displayed in the plain text format. To enable this feature, install the October 4, 2016, update for Outlook 2013 (KB3118367), and then follow these steps.
Important Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
Exit all Microsoft Office applications.
Start Registry Editor:
In Windows 10, go to Start, enter regedit in the Search Windows box, and then select regedit.exe in the search results.
In Windows 8 or Windows 8.1, move your mouse to the upper-right corner, click Search, type regedit in the search text box, and then click regedit.exe in the search results.
In Windows 7 or Windows Vista, click Start, type regedit in the Search programs and files text box, and then click regedit.exe in the search results.
Locate and then select the following registry subkey: