I am able to import emails from more than twenty years ago to the Mac Mini. These were previously imported from backup files of a Mac Performa 580CD computer (A computer I used from 1995 to 1998-1999 that had my first computer mouse.). These mid-nineteen ninety emails were on Zip disk backups of the Performa at one point. I had previously imported these email files to my eMac sometime in the late 2000s. Now generally they are all mbox format files, so will open in a variety of email softwares including Linux email clients.
Now my project becomes loading all the emails I have from the past into the Mac Mini Mail software and then exporting them as one big mbox file. I will test with all the old Performa emails in one mbox file first. Then the challenge will be the Outlook Express files from the early 2000s. I believe I have lost most of my late 1990s emails on an old Windows 95 computer that was wiped. These Outlook email files have mostly already been imported into Mac Mail on the eMac, so this may be simple. Then it is simply a matter of importing more recent Outlook files form 2009 to 2017. I may organize these as one year at a time, mbox files.