type:
variable
name:
$bounce_mailbox_purge_unprocessed
purpose:
Delete bounces from the mailbox that were not matched to be for a campaign or subscriber (unidentified bounce)
allowed:
true false 0 1
default:
true
category:
bounce management

It is best to set this to true. The bounce will be added to the phpList database, so you can view it in the phpList admin pages.