Talk about circular structure, atani, and the jurisdictions of different Ai-Naidar above the Wall of Birth.
The Ai-Naidari concept of mail is very different from the human, in that most Ai-Naidar don't write letters; certainly not in the volume that humans do. Messages exchanged between Ai-Naidar within the city district are passed by that Ai-Naidar walking over and talking to whomever they want to talk to. If they arrive and find the other person not home, they leave a card or message then.
For messages from outside the city district or out of easy walking distance, letters can be walked to the district's mail-handling facility and left there, and then they are carried once a day over to the appropriate facility. Most Ai-Naidar routinely walk to the central facility to check for any letters, but it may not be daily.
Parcels are rarely shipped to and from individuals, who tend to do all their shopping locally, or who wait for traveling Merchants to arrive and sell their wares at storefronts set aside for itinerants, so the mail-service is almost entirely the few letters that Ai-Naidar send out-of-district, or to other cities. Since most Ai-Naidar live with their families and near their intimates, it's rare for them to need a postal system like ours. They focus heavily on what they can reach on their own two feet, and spend a good part of their days walking.
For special messages, there is a Public Servant courier system, paid for by transaction taxes within each district, and which can be used (within reason) by any individual in the district. Couriers are for messages that must be sent with speed or special privacy. These Couriers wear gray short coats, split for riding, and have black sashes.
In addition to the Public Servant couriers that serve a distict's population, Servant-caste couriers are employed by Houses above the Wall of Birth for their particular needs. These couriers wear gray short coats, split for riding, and wear sashes in their House's colors.
Finally, Thirukedi has His own couriers, considered osulked, and sent only by his hand. They wear lighter gray short coats in the same style as other couriers, but wear cream-and-gold sashes around their waists, and up diagonally from hip to shoulder, pinned with the mark of empire; their coats are also piped in gold, as are their messenger bags.
Goods and cargo are not imported into cities by the "postal service," but are brought into and out of cities by the Merchant caste, which has a large percentage of Ai-Naidar who enjoy travel.