For one thing, I have a ton of mailboxes already, and if I have to manually place mailboxes to spawn the correct number of mail vans for the amount of mail waiting, that's a big flaw in the mail system. If garbage trucks can collect trash, mail vans should collect mail. Unless the idea is that people only send mail through public mailboxes--not like real life and unnecessary micromanagement in a game.
Also, all those mailboxes are empty anyway, so either that's not how the mechanic works, or it's broken.
I even followed a van around a high density area out of curiosity. It drove by building after building "red" with accumulated mail, but was only about 2% full before it drove back to a post office. The whole mail system seems to have some problems, and I can't see how it could have been released in this state if it was tested on even a cursory level.