Your email is secure and searchable in macOS Mail — it is available anywhere on the IMAP server and is available for download as mbox file .
Mbox files store emails in a simple transferable format that can be easily imported into other email programs and operating systems.
How to Export a macOS Mail Folder to an Mbox File
Export from macOS Mail very simple. Follow these simple instructions to save an email folder as an mbox file using MacOS Mail: