In SAP FI, if a customer misses the payment for the outstanding invoice by payment due date, you can generate a dunning letter using SAP FI and send it to the customer address to remind him of the outstanding payment.
The dunning system enables to trace liable customers who have not paid their open invoices within a given time span. It enables you to handle the process from, for example, sending a reminder to customers of their outstanding payments through referring such customers to collections agencies.
The dunning system covers the following documents.
Go to SPRO → SAP Reference IMG → Financial Accounting (New) → AR and AP → Business Transactions → Dunning → Basic Setting for Dunning → Define Dunning Keys → Execute.
Here you define the reason for dunning block reason under a key. It can be defined for an item or for a customer master. Blocked item or customer account are not considered for dunning reason.