Chosen Solution
I am upgrading my Mac OS to 10.14. I have lost mail folders in the previous upgrade, and I could not retrieve the files anymore. This time, I have thousands of emails under On my Mac folder. Suggest me something just not to face the old issue again.
There are quite many users like us who have lost Mac Mail folders while upgrading. Time Machine Backup wasn’t much fruitful as the recovered emails were mostly empty. Later I learned, Mac mail doesn’t download the mails to local storage if not opened. Thus Time Machine fails to backup those emails. Currently, we are using Mail Backup X that offers auto-archiving of emails directly from the Mac Mail. I hope that helps you take Mac Mail Backup properly as well. it sets a warning against all the partly-downloaded emails and instructs me how to download those emails properly. That saves my emails for a longer run. Offers a free trial: http://www.mailbackupx.com/how-to-scener…
The best thing is to use Apples supplied back up program called Time Machine. You’ll need an external drive which is at least the same size if not larger that your internal drive. Connect the drive up and then open Time Machine. Review this Apple T/N How to use Time Machine to back up or restore your Mac
