Show or hide the ruler

While composing a message or reply, select  Show Quick Access Toolbar from the Ribbon selection menu.

Select the drop-down menu and select Customize the Quick Access Toolbar > More Commands.

Under Choose commands from, select Commands Not in the Ribbon.

From the list, select Ruler > Add > OK to add this command to your Quick Access Toolbar.

Select  View Ruler to display the ruler above your message content. To hide the ruler, simply select the button again.