How do I ensure correct defaulting of business unit for the sender's or recipient's email?

You can set the defaulting options on the Manage Intelligent Document Recognition Options page.

When a sender emails an invoice to IDR, the email will be defaulted in routing attribute 1 in Payables. Consider the scenario where the sender's email is before the comma and the recipient's email is after the comma, such as this: [e-mail], [e-mail]. If you want the second email to derive the business unit, then you must select the recipient email on the Manage Intelligent Document Recognition Options page.