I recently sent a return receipt certified mail letter to a Texas PO box for a business that I knew was closed over five years ago. I was required to contact them and prove that any correspondence was impossible as noted by the destination PO, which was all I really needed indicated by the undeliverable letter.
It should have IMO been immediately obvious it was undeliverable upon first delivery attempt. I had already gone thru this same exact procedure 4? years earlier. I could do nothing until I received the undeliverable mail back.

I tracked the certified mail daily. It showed the letter in an effective daily loop of being out for delivery. My local mailing PO could do nothing. I attempted to initiate a lost item investigation. 4 weeks after I mailed it originally, or 25 days after it arrived at destination, I got my undeliverable letter back.

Two weeks later the PO emailed they were still looking for my letter. eyes

Certified is not as secure as one would think it appears.

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.