That was way before the Catalina Apple Mail data loss bugs. 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.
It’s been happening since 2016 (first time I noticed) and last reporter to them in January 2021. Then I gave up. It still happens.
Anyway that’s just one immortal Mail.app bug.
I am just in the process of finding another mail client (preferably native, paid, and open source that doesn’t read my mails)
That was way before the Catalina Apple Mail data loss bugs. 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.