Use the sidebar or Favourites bar in Mail on Mac
Show, hide or change the Mail sidebar
The Mail sidebar shows 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 Mailbox List or click Mailboxes in the Favourites bar.
Resize the sidebar: Drag its separator bar left or right. If you drag it all the way to the left, it hides the sidebar.
Hide the sidebar: Choose View > Hide Mailbox List or click Mailboxes in the Favourites bar again.
To change the size of the sidebar icons, choose Apple menu 
> System Preferences, click General, then select a sidebar icon size.
Show, hide or change the Favourites bar
The Mail Favourites bar provides quick access to your favourite or most-used mailboxes and folders.
In the Mail app on your Mac, do any of the following:
Show the Favourites bar: Choose View > Show Favourites Bar.
Add a mailbox to the Favourites bar: Drag the mailbox there from the Mail sidebar. (Mail doesn’t actually move the mailbox; it adds an alias for it to the Favourites bar.)
Remove a mailbox from the Favourites bar: Drag the mailbox out from the Favourites bar; visual and sound effects indicate it’s been removed.
Hide the Favourites bar: Choose View > Hide Favourites Bar.