I believe it is all flat-file. When I say "Database" I am just referencing what they call it:

    -d DB_DIR, --db-dir DB_DIR

But from what I see the structure looks like this:

    db/
        YYYY-MM/
           1234554543262346.eml.gz - I assume the meat and potatoes of the email along with attachments, not sure
           1234554543262346.meta - JSON file with msg_id, thread_id, flags, labels, subject, etc