this part here...
dovecot uses Maildir format (not mbox), so if you install ISPConfig on the server, please make sure you enable Maildir under Management -> Server -> Settings -> Email. ISPConfig will then do the necessary configuration.
If you do not want to install ISPConfig, then you must configure Postfix to deliver emails to a user's Maildir:
postconf -e 'home_mailbox = Maildir/'
postconf -e 'mailbox_command ='
are we supposed to run this if we are going to install ispconfig on the server?
or is this telling us what to do IF we are NOT installing ispconfig...