Mail for Mac: Import or export mailboxes
Mail can import messages from other email apps. It can also import mailboxes that you exported from Mail to use as backup copies.
Choose File > Import Mailboxes.
Select a source in the list, read the information that appears below the list, then click Continue.
Some options require that the app you’re importing from is active.
If you’re importing a mailbox exported from Mail, or a folder (such as POPfirstname.lastname@example.org) from the ~/Library/Mail/V4 folder on another Mac, select Apple Mail.
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.
If the email app you’re switching from isn’t listed, click Cancel. Consult the documentation for the other app to learn how to export messages in mbox format (most email apps can). Then import the mbox files.
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.