Import or export mailboxes in Mail on Mac - Apple Support
https://support.apple.com/guide/mail/import-or-export-mailboxes-mlhlp1030/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.
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