Bounces doesn't work!

I’ve configured Phplist (bounce and mail parameters).
I’ve no error and no bounce info !
When I display bounce stats, I’ve a clean page with no error message !
Here are my paramaters :
$bounce_protocol = ‘pop’;
$bounce_mailbox_port = “993/imap/ssl/novalidate-cert”;
bounce mail box receives mail deliveries with no problem.

I tested mbox unstead of pop us bounce_protocol but no result !
I hope you can help me …