A Dunning Letter is a formal document sent to customers who are behind on their payments. It informs the customer of the amount they owe and provides them with details on how they can make the payment. The Dunning Letter serves as both a reminder and a warning, urging the customer to make their payment or face consequences such as late fees, collection agency involvement, or legal action. The tone of the letter should be friendly yet firm, so that the customer understands the seriousness of their outstanding debt.