Week 1: watch the rejections, every day
Set a fixed time each day, say 10am, to review what was sent and what was rejected. Most first-week rejections trace back to data: a customer’s TRN typed with a space, a missing buyer electronic address, an item with no tax category. Fix the master record, not just the invoice, or the same error returns tomorrow.
Keep a simple log with the invoice number, the reason and the fix. By Friday you will see a pattern. A Dubai wholesaler might find that 18 of 22 rejections in week one came from four customers whose records were created before the clean-up. Fixing those four records removes most of the problem.