My Mail Rooms are currently overflowing with Unsorted Mail, and I haven't been able to stop it. I currently have 7 Mail Rooms employing 5 Prisoners each, and the prison contains around 650 Prisoners. My  Delivery areas are filled with mail, and more comes every day. While the Prisoners are receiving their mail, the amount never seems to diminish.
I have looked online and asked people, but no one seems to know how many Mail Rooms I should provide.
How much mail is received per Prisoner?