Which OS, SQL, Postfix and Dovecot files/folder need to be backup for disaster recovery?

I'm running an e-mail server (Postfix/Dovecot) on a Pi4, Buster OS. We want to setup a parallel Pi (same basic image) and use rsync to transfer critical files to this Pi using a cron job on a regular schedule (daily, hourly, ...). I know that the mail content files (/var/vmail) and the SQL files (/var/lib/mysql) are important, but are there other critical e-mail server files which need to be transferred? The goal here is to have a "backup" server up-to-date, ready and configured if the primary server has a serious failure.