Tip: To show or hide the Mail sidebar, choose View > Show Sidebar or View > Hide Sidebar, or press Control-Command-S. To show or hide the Favorites bar, choose View > Show Favorites Bar or View > Hide Favorites Bar, or press Option-Shift-Command-H.
Open Mail for me
Go to the Mail app on your Mac.
Do any of the following:
In the Mail sidebar: In Favorites, Control-click a mailbox, then choose Remove from Favorites.
In the Favorites bar: Drag the mailbox out of the Favorites bar; visual and sound effects indicate it’s been removed.