Your question reminded me of the postal experiments conducted by the Annals of Improbable Research. If nothing else, they’ll keep you entertained until someone who knows what they’re talking about comes along.
The USPS processes 200 billion pieces of mail per year. 5.5 billion (3%) aren’t deliverable as addressed – 1.3 billion are returned to sender, 2 billion are discarded, the rest are forwarded.
While those numbers don’t specifically deal with “lost” mail, it looks like about 1% are thrown out.
Personally, most of my mail is bill payments, and I’ve had only one lost in 15 years – it came back a few months later as just a mangled half of a check, someone at the USPS must have found it and returned the portion with my name and address on it.
Every once in a while you read news stories about mail carriers who got tired of lugging so much mail around every day, and they have years worth of old letters stashed in their basement, etc. But overall, I think the post office does a pretty good job of getting letters where they need to be in a reasonable time.
I once got an unimportant domestic business letter more than a year after the postmark. On the front, someone had used a rubber stamp to imprint a message like “found between a shelf and the wall.”
I presume the USPS didn’t get such a stamp made if such a thing happened once every 10 year.
Last year a library in a nearby town was remodeled. The building had been a post office back in the 1950s I think. Anyway, during the redo, someone found a letter postmarked during WWII from a service man in Europe.
Also, my experience is that you can reduce the chance of your mail being in that 1% that is undeliverable & unreturnable to sender.
The biggest improvement comes from improving the address you put on the envelope:
make sure it’s accurate. USPO employees get pretty good at guessing what street you mean from your spelling, or guessing if it’s North or South on that street, but delivery odds are better if they don’t have to guess. See here to get their USPO version of the address.
make sure it’s legible. Computer printed or typed is better than handwritten, printed handwriting is better than cursive (script) handwriting, and neat & readable is always better.
make sure it stays on the envelope. Adhesive labels with worn-out glue will fall off, and then where do they deliver a blank envelope? Also window envelopes with the address not showing thru the window – they’ll probably get delivered – eventually, but not in a timely manner.
as a last resort, return address labels help – at least it will get back to you if one of the above things happen. Then you can fix it and resend it.
People have mentioned that bill payments hardly ever get lost. Note that they comply with all of the above: pre-printed addresses, even barcodes, done in a typeface that can be machine-read, located in the proper place, etc. Companies work to make sure their payment envelopes don’t get lost or delayed!
Much of this “lost” mail is mail that is delivered to the wrong address, and the recipient never bothered to call the PO’s attention to its error or attempt to deliver it to the addressee. In the past few years, I have received about a dozen envelopes not addressed to me. In many cases, the addressee was a neighbor, so I delivered it to the proper addressee. In the other cases, I wrote on the envelope that it was misdelivered, and dropped it in the mailbox. Sometimes these errors occur because the mail was very thin and easy to be picked up with another envelope to be delivered as one mail. Other times, I cannot account for these negligent acts by the mailperson, other than haste and sloppiness. I’ve sent mail (and in one case a check for payment of a credit card) which were never received by the addressee. So I suspect those were misdelivered.
Years ago, these errors never, or very rarely, happened.
(BTW, one credit card company, which was in fact my friendly credit union, would not waive the interest penalty, so I stopped using its card. I cut it into pieces and sent it to the credit union. I was especially irate because a few years previously, a check I sent to a bank credit card company was never received, and the bank agreed to waive the interest since I’ve always paid on time.)