Go to Settings -> Communication -> Email Templates. Select the appropriate category (e.g., Appointments to select this template when sending the flash mail) and click [+] in the top right corner.

Click to modify the template. The editor is displayed here. On the right, the various options are displayed.
As an example, we will add a 'Text' module.

Click 'Mobile' in the top right to show and modify the mobile view.
We will now add a 'Divider' module to add a line under the heading.