An easy way is to use an IMAP client like the macOS Mail.app that syncs raw email + metadata in .mbox format. You can find the files in ~/Library/Mail/V4. Combi

An easy way is to use an IMAP client like the macOS Mail.app that syncs raw email + metadata in .mbox format. You can find the files in ~/Library/Mail/V4. Combine with Time Machine for extra backups in case you’re worried that your mail server decides to clear your local copy over IMAP.