Jul 22, 2020.
Mail User Guide
Mail can import email messages that have been exported in mbox format from other email apps, and import mailboxes exported from Mail on a Mac. You can export mailboxes in mbox format.
Import mailboxes
In the Mail app on your Mac, choose File > Import Mailboxes.
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/V7 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, 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.
Reorganize the imported messages, if you like.
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.
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Export mailboxes
Apple Messages For Mac
In the Mail app on your Mac, select one or more mailboxes, then choose Mailbox > Export Mailbox.
Choose a folder or create a new folder, then click Choose.
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.