H1b denial notice letter receive date proof

[ANarayan]

Hi, My H1B visa extension got denied on 12th but have not received the denial notice till now, its been more than 10 days. As because the denial notice is not yet received, i have not left the country till now. Also, Planning to apply for new petition under consular processing, but when i go for visa interview later how do i prove that i received the denial notice letter late, that is why i left the country late or simply, how to prove the receive date of H1B denial notice letter, not the denial date but the notice received date. Any suggestions/advice is appreciable. thank you.