There is something bit confusing in your message: You said that emails "new' PO's from production were approved!" were sent. Normally RSWUWFML2 is used to sent about work items that are waiting for completion. So in your case I would be expecting to get emails "There are POs waiting for approval". Can you see my point? Are you sure that RSWUWFML2 is responsible of sending the 10.000 emails?
If it was the RSWUWFML2 that sent the emails, you might have had an issue with the field "Job suffix" in the variant. In most cases the idea of RSWUWFML2 is to send emails of new work items. So each time when the report is executed, it leaves a time stamp in the system, so that it knows when was the last time that the report was executed, and then it takes into account only the work items created after the last execution. Now the tricky part in this is that the time stamp has the key "job suffix". So, if you have changed the job suffix in the variant, it might be possible that the report does not find the "correct" time stamp, and consider all work items as new...
Regards,
Karri
PS. After client copy it might be a good idea to ask basis to turn off the email sending job, so that the emails will not be sent from the queue unless you really want it...