>* I think SyneRyder means what format are you storing emails on disk?*

Yup, that's what I wanted to know. Since Thunderbird & Postbox both used the same Mbox Unix format for my 20 year email archive saved on disk, it was very easy to switch between them. And since it's an industry standard, I'm reasonably confident I could export/migrate it to a new email client when I have to - or find someone who has written software to do it.