My 83b election may have been lost in the mail, what rights to I have to prove to IRS I sent this on time?

I mailed in my forms for my 83b election within 30 days of exercising. I did this via an automated postal service machine, and purchased a return receipt and certified mail. I put the stamp on the letter and have a photograph on my iphone showing the letter with the stamp on it (and certified mail barcode) which may have encoded the location and time the photo was taken, and have a copy of the receipt that the postage was purchased. I put the mail in a postal box. I clearly marked my return to sender address on the mail. Since I did not receive the mail back and paid for "Certified Mail" I assumed this had been delivered to the IRS. The post office now says they have no record of this letter being mailed. How can I prove to IRS this was mailed?