output queue automatically creates email from spool file

We have a client with an output queue which is supposed to function as
follows: any spool file placed in that queue gets converted to a pdf and
sent as an attachment to recipient(s).
However, spool files intended for certain users never receive their email
and the spool file gets changed to a HLD status.

My presumption going in was that there would be a data queue associated with
the output queue that receives a data queue entry when a spool file arrives
in the RDY status. So, I would look for that program that is monitoring
that data queue, right? Nope. The WRKOUTQ display indicates the output
queue has no associated data queue.

So, is there some OTHER way that a process could be monitoring an output
queue for a spool file arrival? I could imagine a never-ending program that
somehow wakes up in some cycle and calls an API to lists the entries in the
output queue and goes from there, but why would anyone write that? Data
queues associated with output queues have been around forever. Perhaps I am
exposing my ignorance here, but I'm always ready to learn.

BTW, the client knows of no non-IBM vendor software owned that would perform
this function.

Thanks

Tom Garvey