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. 
I like this variant as it's quick and dirty. I do not yet understand those listeners that well. But I do know dovecot... so here is the variant I'm using:
this is the normal namespace with removed prefix.
Below that you can add another namespace:
Could that be incorporated to this listener? Then anyone could use it even when coming from Courier.
Background story: actually Mac OSX Notes.app has a problem with prefixes on IMAP sync. So I looked out to find a way to satisfy both worlds, those with and without prefix set.
I use a listener file with exactly the same namespaces, how you described it here.
Maybe I'll expand it a bit to be more flexible and then I could share it.
The attached listener file 10_dovecot_namespace.pl changes the namespace back to dovecot default:
and also creates a new namespace entry as backward compatibility for Courier IMAP:
For more information about this options please read http://wiki2.dovecot.org/Namespaces
Usage: Copy the file to your server into the /etc/imscp/listeners.d/ directory and remove the .txt extension. Run the setup engine to activate it immediately:
/var/www/imscp/engine/setup/imscp-setup -dn
Thank my friend 
All your listeners are welcome