About seven years ago, I was at a sushi bar and struck up a conversation with an older gentleman sitting next to me. He told me he was a developer and created systems for USPS. I am always fascinated by the technology used in large scale systems so I picked his brain for a good hour.
From what I recall, he said at the key distribution centers, USPS scans every single mail (in standard envelop sizes) and in under a second, runs OCR for the destination address. Results from OCR are matched to the address database and if the match is significant, the mail is automatically diverted to the correct queue. Now here's the fun part - if OCR fails or print/handwriting is unreadable, a photograph is immediately sent to one of the hundreds of humans waiting to decipher the address and type it in (think Amazon Mechanical Turk). The humans have under 10 seconds to read, decipher, type, and submit the correct address. During this time, the letter is held up in a waiting buffer and the moment the correct address is available, it is diverted to the correct queue.
I asked him if that means USPS took a photo of every single piece of mail and he said yes, they had to, otherwise nobody would ever get any mail due to the sheer volume of mail they had to manage. I asked if the photos of envelopes were saved forever and he said, well, I'm pretty sure they are but I'm not allowed to publicly admit that.
I know it's a personal anecdote but that was seven years ago. I can't even imagine what they're doing now.