I believe that blue line is a consequence of a setting related to whether you choose to display messages in groups. Most people seem to like groups. I find that organizing messages into groups complicates navigation when I interact with messages using only the keyboard (e.g., Up and Down arrow keys; Shift + Up, Shift + Down; etc.) But to each their own. Here's how to change the setting:




View > View Settings > Group By...


Uncheck "Automatically group according to arrangement"

Thank you Alan Cantor, much appreciated. I haven't adjusted my Group Settings so I am not sure that is the cause. I agree that removing the Group Settings removes the vertical lines though but I rather like my Group Settings.