You can export and save all your Apple Mail mailboxes data items using the help of below mentioned steps:-
First Method: Export mailboxes and Save Offline Email Data
- Choose one or more mailboxes, and then select Mailbox > Export Mailbox.
- Select an entirely new 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.
Second Method:-
You can go directly to the folder where Mail saves its data through the Finder and duplicate it to store a copy elsewhere. Inside the Finder, menu Go > Go to Folder and then paste this path:
~/Library/Mail/V5
And the last option you may go for some reliable and trustworthy 3rd party application that might be available in the apple mail backup.
Let me know if this was helpful. Thanks