Mail app where is imap account saved /cached on disk?

Can someone explain where the mail app saves its imap data? I do no see in in the data directory.

And is there a way to force download imap mail from server? So not only when I scroll down, but all in one run?

I run Nextcloud 19 on an Ubuntu 18.04 server.

No one? Is it stored in the database, or in the filesystem?