Opening .mbox files
After following the directions for offloading emails to a separate hard drive, I find that the .mbox folders on that drive do not contain any items I can open.
Mac mini, macOS 14.6
Question marked as
1. On your Mac, open the Mail application.
2. In the File menu, select Import Mailboxes.
3. In the new pane that appears, select Files in mbox format from the option list, and hit Continue.
4. In the next window that appears, navigate to your previously saved MBOX file, select it and hit Choose.
5. In a new window, Apple Mail will confirm where you can find the newly imported MBOX file, and you can select Done to close the window.