Inbound Mailer Processing

We've recently begun to use the Notification Mailer to process Purchase Requisition Approvals and things have been going fairly smoothly.  However, we've had several instances of users Approving a Req (via the Approve link in the notification email that generates an email message to the inbound mailer) and then the email that gets sent to the workflow mailbox ends up being discarded for no apparent reason.  There is also no notification returned to the approver so no one knows that this has happened unless someone is checking the status of the req.  If the user attempts to approve again (using