Assuming mail means USPS, you might try this:
https://www.usps.com/help/missing-mail.htm
I can tell you as a small business owner, I think I've had more mail issues this year than in the 10 previous years combined, and things are just getting crazier now. I've had UPS show up in personal cars and a couple of nights ago I got a delivery by UHAUL truck at 9:30PM. Good luck!