The postal service routing system developed into a highly fault-tolerant system, by necessity. You may intentionally omit required information, but the system

The postal service routing system developed into a highly fault-tolerant system, by necessity. You may intentionally omit required information, but the system doesn't know that. It could also have been obscured, damaged, etc. The machines try as hard as they can to get the letter as close to the destination as possible and then a human takes over. Unless your local postal carrier is a jerk, it is highly likely to succeed. Even in NYC/LA/Chicago: the postal carrier has a small area they are responsible for, they are just as likely to know all the names as a postal carrier in a small town.