Where does the payment acknowledgement email come from and how to customize it?

Summary:

We recently set up a payment acknowledgment with the bank and have the payment file register with document detail email once the disbursement and acknowledgment are completed. The email that comes is very undescriptive and does not have a clear subject line or report name showing exactly what is being sent. Our users get a lot of emails so this not a great way to send information. Many of us have looked and tested to see how this is being sent, and we can't find anything. We would like to customize the email, subject, and report