I have some very large .mbox files exported from Mail and other applications (unix mail, Eudora, Thunderbird, etc.) over the years/decades. Is there any way, on macOS Catalina (e.g. 64-bit), of directly reading the mail in these .mbox files without importing them to Mail? (as these archives can be larger than the available storage space on my local User account partition)?
You have to use a third-party app, on MacOS I haven't found any free ones (on Windows you have some free). This one for example: https://apps.apple.com/us/app/mbox-office-viewer-migration/id829623734
All messages in an .mbox mailbox are stored as plain text so you can also open it with any text editor.