Experiencing same problem. Added 3 misspelled email addresses manually and processed queue.
Have tried the options outlined with settings:-
$message_envelope = 'firstname.lastname@example.org
$bounce_protocol = 'pop';
$bounce_mailbox_host = 'pop.domainname.co.uk';
$bounce_mailbox_user = 'email@example.com
'; (correct user)
$bounce_mailbox_password = 'listmail';
#$bounce_mailbox_port = "110/pop3/notls"; (tried both)
$bounce_mailbox_port = "110/pop3";
php_safe_mode is set to off.
No bounces appear to be identified and none are in mailbox. Is there another setting I'm missing?