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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.
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In the Mail app on your Mac, do any of the following:
Show the sidebar: Choose View > Show Mailbox List, or press Shift-Command-M.
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 Mailbox List, or press Shift-Command-M.
To change the size of the sidebar icons, choose Apple menu