[nobody9999]

>The best arguments for a long-term mail storing solution - it keeps your emails and all your mail settings, in a form that is truly portable, so that you can easily move it from computer to computer

Absolutely. I have emails in my Thunderbird profiles going back to 1996. No one can look at them unless they hack my systems and/or have physical access to them.

I highly recommend Thunderbird (download it now. You won't be sorry.)[0].

[0] https://www.thunderbird.net