I really don't recommend doing that with Apple Mail. I did, many years ago, and ended up losing quite a few emails due to Apple Mail bugs; I did it again a few

I really don't recommend doing that with Apple Mail. I did, many years ago, and ended up losing quite a few emails due to Apple Mail bugs; I did it again a few years later and ended up with weird corruption of email headers (wrong "received" dates). That was way before the Catalina Apple Mail data loss bugs. https://mjtsai.com/blog/2019/10/11/mail-data-loss-in-macos-10-15/ https://mjtsai.com/blog/2019/10/11/mail-data-loss-in-macos-1... Any sysadmin will tell you Apple Mail has had occasional weird dogshit bugs for a decade. Use Thunderbird, it's more likely to be robust and not corrupt your emails, then feel free to switch back to Apple Mail once you've moved everything.