Lost USPS tracking number

In July I had to return my defective cell phone. I actually got 3 packages before I got the right device. I sent all 3 boxes back and received conformation by email August 5,2015 the defective device had been received. I've gotten another email saying I hadn't sent my defective device back. I've misplaced the tracking information from the USPS. What can I do to make sure all three boxes have been received?