Bird Messenger Network

While magical means of communication are possible in Altwaus, the rarity of those who can perform the magic required to achieve magical communication (not to mention the haughtiness that tends to accompany such abilities) means that the vast majority of long-range communication in Altwaus is instead done via messenger birds. 
The exact species of bird used varies with the culture and nature of the message, but common choices in [The Thirteen Kingdoms include pigeons, doves, crows, and owls. Some more exotic species are sometimes used as well, but that would typically be only for the more affluent.
Typically each bird has a "home" roost where they are instinctually bound to return to, and these birds are then caged and physically delivered to nearby settlement. Then, when that settlement has need to send a message, they affix their message to the bird's leg and release it - trusting it's instinct will take it back to it's home where the message can be received.

Access & Availability

All major settlements will have at least one aviary available for public use, with larger settlements and cities having dozens of public and private aviaries sending and receiving messages at any time. The public can access these services in these cities at a cost that varies depending on demand and distance. 
Smaller settlements are less likely to have aviarys open to public use, and the only aviary is typically located at the stem of the city hall or place of worship. In these instances, messages are typically reserved for great importance and letter carriers are used for the vast majority of communication, as the settlement can be days or spans without a bird available to them once it has been released.

Complexity

The messenger network consistents of various sub-aviaries that funnel into prominent hubs that organize and redirect messages further down the line. In the event a relay bird arrives carrying a message, the works at the aviary take the message (unsealed of course) and attach it to a new bird that will take it either to its destination or to a destination further down the chain towards that. 
While many of the major relay hubs are located in or near major settlements, the vast expanse of Altwaus means that there are frequently aviaries atop mountains or cliffsides in the wilderness. Often small settlements have sprung up around these more remote aviaries to service the needs of the workers and their families - leaving to strange little settlements seemingly in the middle of nowhere.