These signs can be found throughout Temuair at the different religious temples; outside the Mileth justice hall; or on the side of the road.
The signs allow you to send and read messages to and from Aislings. If you double click on the message, a window will pop up:
Most of this is self-explanatory, but here's a quick introduction. To send messages, you click on New. New messages from other Aislings will appear in yellow on your screen; a selected message, in blue. Double clicking on a message will open it for you, allowing you to read it.
You can also send personal mail. To do so, go to any shop in Mileth, and click the bulletin icon. The look and feel is the same as for the bulletins.