how to export emails before downgrading to sierra

Hi there,

I would like to kindly ask for helping hand.

I am running High Sierra already for one month. High Sierra makes me crazy. I do not like the system at all. Full of bugs. With Sierra was everything running smooth and easy.

Therefore I will downgrade to Sierra.

Before that I would like to kindly ask for helping hand how to export all the income and sent emails, and how to import them again after downgrade to Sierra. (Just because the latest backup with Sierra will not include the emails newest emails, of course).

Thanks in advance.