How to turn on / off the ruler in Outlook 2013 / 2016?
Software | Microsoft Office
Click on "New Email".
Click "Customize the Quick Access Toolbar" and click "More Commands".
In the Choose commands from list, click "Commands Not in the Ribbon".
Select "Ruler" and click "Add".
The ruler is added to the Quick Access Toolbar on top.
Now you can turn on and off the ruler.
Software, Microsoft Office, Outlook 2013, Outlook 2016, Ruler