"Who can be the most trustworthy person you can find? Not your brother, your spouse, or your friend. The Courier. You trust us to deliver your words to others, and you trust us with your secrets. But we earn that trust."
Under the Guildmasters are Tradesmen, who delegate Journeymen to deliver the letters, and the Apprentices to deliver news to neighborhoods within districts.
Journeymen deliver the letters and packages to individual homes after memorizing routes from their Apprentice years.
Apprentices spend years memorizing their districts and neighborhoods to learn the fastest routes. Over time, Apprentices are trusted to deliver simple messages to areas as tests before they can advance into Journeymen, but older Apprentices teach the younger ones as well.
The courier service has packages and communications sorted out by Journeymen and Apprentices who are either temporarily or permanently unable to carry deliveries.