There are several ways communications are sent and received on Martapa. Here are the major ways.
1) Letters brought by runners. There is a whole network of runners not only in Tridon, but Ariella as well. Stations are spread out for rest areas and supplies for the runners so they don't have to carry a lot of supplies and thus can run faster.
2) By falcon. Nobles keep falcons to fly from known location to known location. They can carry small, short messages which can be the beginnings of communications.
3) Magical means. The use of mirrors to speak through, mages can talk to each other. Though there are lots of mages, only a few are capable of doing this. And it is straining, so only the rich can afford to have such means of communications.
4) And what we would refer to as "snail mail". Letters can be written and brought by stage or boat to the next location until it reaches its destination sometimes months later.