I just came across a great New York Times piece on the mail service in the Victorian era. It's full of trivia: for example, did you know that in 1889 London, mail was delivered TWELVE TIMES A DAY? Read this:
In Victorian London, though service wasn't
there’s locked mail boxes here. like at apartments with a shared lobby. the owner just gets a key.
This, and the outgoing box is locked too. Anyone can put mail in, only the postie can take it out