SOLVED Bounce configured but still not working

I configured phpList to handle bounces. But when I make it send to an address that I know should bounce, it does not.

We’ve installed v3.3.1 and these are the settings I’m using:

$bounce_protocol = 'pop';
$bounce_mailbox_port = "110/pop3/notls";
$bounce_mailbox = '/var/mail/listbounces';
$bounce_mailbox_purge = 1;
$bounce_mailbox_purge_unprocessed = 1;
$bounce_unsubscribe_threshold = 5;

I’ve verified that the user connects to the mail server, as the Process Bounces dialogue says “bounces to fetch from mailbox” instead of producing an error.

UPDATE: I tried a different test address and it worked fine.