I would take a closer look at your workflow.
My guess is that the read receipt is being sent to WF_INITIATOR which in your case happens to be WF-BATCH. There are various reasons why WF-BATCH is the initiator.
1. If the Workflow is triggered by some batch job which is executing under the User ID WF-BATCH then this will set the WF_INTIATOR container element to WF-BATCH.
2. The other possibility is that if the workflow is triggered asynchronously and a logical destination is specified then the User ID that is used to log into the logical destination could be set as WF-BATCH.
I would take a look at the Workflow and create an attribute to reference the User ID of the person who created the purchase order.
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