Bounce testing problem

I’m trying to test my bounce handling config by send a campaign to a list that contains a non-existent address. When I process the queue, phpList gets into a loop repeatedly sending the campaign to the ‘bad’ address.

I can see by monitoring the server mallog that the sever is rejecting the mail with “550 5.1.1 Recipient address rejected” but phpList immediately resends the mail to that address and does not move forward in the queue.

I’m on Centos 7 using PHP7.4 and Postfix / Dovecot