If you use Dovecot on a standard installation on Debian, the prefix is not set.
With i-MSCP the prefix is set to INBOX. because Courier use it as standard.
I personally do not like that and on the basis of this topic Dovecot Namespace und Quota , I wrote a listener.
I will use in future no Courier, so I don't care if there are problems with the prefix.
Everyone thinks the same way, can use this listener.