Import or export mailboxes in Mail on Mac
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.
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/ 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 select 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.
Reorganise 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.
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.