Use the sidebar in Mail on Mac
The Mail sidebar provides easy access to mailboxes, folders and flagged items for the email accounts you use in Mail.
In the Mail app on your Mac, do any of the following:
Show the sidebar: Choose View > Show Sidebar, or press Control-Command-S.
Resize the sidebar: Drag its separator bar left or right. If you drag it all the way to the left, the sidebar is hidden.
Hide the sidebar: Choose View > Hide Sidebar, or press Control-Command-S.
To change the size of the icons and text in the sidebar, choose Apple menu > System Settings, then click Appearance in the sidebar (you may need to scroll down). Click the Sidebar icon size pop-up menu on the right, then choose a size.