1. Click the Mail app on your Mac to open up the application. In the top toolbar, you will see "Mailbox"; select this.  From the drop-down, you will be see "Export Mailbox", click on this.

       2. Choose a folder or create a new folder to export your mailbox to. Then click "Choose".

       3. Mail exports the mailboxes as .mbox packages. If you previously exported a mailbox, Mail doesn’t          overwrite the existing .mbox file; it creates a new .mbox file, such as My Mailbox 3.mbox.

Import mailboxes

1. Click the Mail app on your Mac to open up the application. In the top toolbar, you will see "File"; select this. From the drop-down, you will see "Import Mailboxes", select this. 

      2. Select a source in the list, read the information that appears below the list, then click Continue.

• If you’re importing a mailbox exported from Mail or a folder in the ~/Library/Mail/V6 folder on another Mac, select Apple Mail. To get to the Mail folder on a Mac, click the desktop to make sure you’re in the Finder, press and hold the Option key while you choose Go > Library, then click the Mail folder.
• If you’re importing messages from a Windows or UNIX computer, select “Files in mbox format,” then locate the folder containing the files.

      3. Reorganize the imported messages, if you like.

1. The messages are in a new mailbox called Import in the Mail sidebar. You can drag folders and messages from the Import mailbox to new or existing mailboxes, then delete the Import mailbox.