Here are some searches that we use to find cases which have been closed as spam:
status:"Closed (spam)" type:case
.. adding the type:case returns 135 cases per page, instead of the usual 45.
And just in case anyone else might find this useful (more in a customer care shop situation), here're some searches for finding Mail Delivery Failure notification emails:
(title:"Undelivered Mail" OR title:"delayed 72 hours" OR title:"delayed 48 hours" OR title:"delayed 24 hours" OR title:"Warning: could not send") status:"active" type:case
(correspondant:"Mail Delivery" OR correspondant:"postmaster") status:"active" type:case