Using Classic Outlook on Windows for personal email, calendar, and contact management
The original issue with the Horizontal line not covering the width of the email was fixed in all the recent builds of the different channels on 1/31/20. Please make sure you have the latest build. In Outlook click File, Office Account, Update Options, Update Now.
This page shows the latest builds by channel. If you are on the latest of the respective channel you should not see the issue. You can see your version and channel by clicking File, Office Account under About Outlook.