Obviously not applicable in San Francisco.
Where are you going with that? Never mind, that stuff applies everywhere. Anyway, where I grew up in SF, in the Mission District, our mail slot was about 6 inches off the ground in our front door entrance at sidewalk level, the door was almost all glass except the very bottom. (No security bars, no crime, doors unlocked during the day.) We and the neighbors never had problems with mail delivery. Guess those days are gone. Some postal carriers are plain lazy nowadays. I've seen them drive their little vehicles 25 feet from house to house delivering stuff, crazy. Others park at the end of a block and walk several blocks delivering, and walk back to their vehicle so it isn't all carriers.