A mailbox is like a folder, used to store and organize email messages. Each email account you use in Mail has a set of standard mailboxes—for example, Inbox, Sent, Drafts, and Trash. The Mail sidebar shows the mailboxes for each account, as well as mailboxes you create. If you don’t see the sidebar, click Mailboxes in the Favorites bar or choose View > Show Mailbox List.
Mailboxes on your Mac are local, meaning you can access them only on your Mac; they’re shown in the On My Mac section of the Mail sidebar. Mailboxes on an account’s mail server can be accessed on any computer where you use the account; they’re shown in the account section of the sidebar.
In Mail, you can create what’s called a Smart Mailbox. Using criteria you specify, a Smart Mailbox displays any message that matches the criteria, regardless of the mailbox where the message is stored.