Email backup strategy

I have mutiple icloud, gmail and private domain email accounts where I file all my mail “on my mac”
What would be a good backup strategy?


Time Machine should be backing all those accounts, since the emails are stored locally on your Mac, I assume. However for more than a decade I have been using the Mailsteward app to make a separate backup of my emails. It stores the emails (and attachments, if selected) in a searchable mySQL database. I regularly save the database file onto a bluray disk for long-term storage.