Today we are sure that the mail will be sent every day to our door. But in the early days, no one could be sure about where—or when—the mail would arrive.
There is a stump (树桩) of a big tree in the state of Washington. It was once a "post office". Because people needed a place for the mail carder (信使) to leave their letters, they found a tree at the cross of roads and cut it down ten feet from the ground. Then they hollowed it out and covered it with something. Inside, they put many boxes. Each box had a family’s name on it. The mail carder could leave letters there for everyone.
Even earlier, when there was no post services, people gave their letters to any traveler going in the right direction. Often they gave them to a traveling shoe maker. The traveler might stop in a small hotel and he would leave the letters there. But the letters stayed there until the person to receive them happened to come by and stop at the hotel.
Today an airmail letter can travel across the world in much less time than that. And you know that your letter will go where you want it to go, and when. What does "hollowed it out" mean in "Then they hollowed it out..."