I’ve run a few tests with the mail system and the main issue I encountered is that in the endgame, the post office produces less local mail than investor skyscrapers consume within a single town hall radius.
When you place a town hall and a post office next to it, and you place level 4 and 5 skyscrapers within the town hall radius (only town hall radius, not even the full post office radius), and you supply those skyscrapers with local mail, regional mail, tapestries and fur, you end up with a population that consumes more local mail than it produces. Even without any overseas mail and without any town hall items to increase the population, they already consume more mail than they produce.
It seems that each post office is capped at a max of 3100% which will be reached at around 15,500 residences. At around ~30,000 residences within the town hall radius (which you easily reach with investor skyscraper level 4 and 5), they will consume more local mail than they produce. This completely defeats the purpose of the mail system in the endgame and it breaks the entire mail system chain for creating regional and overseas mail.
The cap of 3100% should be removed from the post office.