Why are you using soulless 'mail system' instead of analog smoke signals and good old carrier pigeons?
Letters mail is fast, but this dependence on mail systems is a single point of failure:
training pigeons(henceforth referred as 'Avian carriers') is fairly cheap and provides a long distance alternative to mail.
To provide redundancy, several avian carriers can be sent with duplicate letters from different routes.
Smoke signals are actually faster than mail but require exclusive access, a trained smoke signal repeater needs only a minute to send a message forth,
with high places commanding a greater range for transmission. The skills are invaluable for sending quick messages while your mail is snails pace.
Avian carriers are the better solution: not as fast, but have a greater bandwidth capacity in single letter.
Plus you can't bribe them, unlike people.