My dad was a letter sorter for the Post Office. These days it's all done by machine.
No it isn't. There are still many letters that defy the most sophisticated machinery.
I hand-sorted letters at RM until I retired. Once there was one address which had obviously been written by a blind person (blind people either keep a finger space between lines, or press hard so they can feel the impressions on the paper. The hand had slipped and parts of the address were written on top of others. I took it to a friend who managed to decipher the street name, looked it up on the RM computer, found the phone number of the occupant, rang her to make sure she was the right person, and sent her the letter in an official envelope. Job done. (it was from her blind friend in Australia)