Post boxes are kind of like routers. You can hand a message to them and they will pass it on to further nodes in the system until it is delivered to its intended destination. With routers messages do not always seem to take a very direct route. If you trace a simple ping to a website in the US it sometimes bounces over to the UK or Europe first.
The beautiful Edwardian router above is just around the corner from my house. Unfortunately I can not use this one as someone decided to vandalise it. These red boxes were installed by the British in the first few years of the 1900’s and painted green after Irish independence. You can still see the royal insignia on it (ER VII).
I wondered what route a letter posted in my own local post box would take so I posted a GPS tracker to myself. I posted the letter at the nearest working post box which is about 2km from my house. Over the next 24 hours the letter ended going on a 230km round-trip via Athlone before circling back to my house!
As I said, post boxes are kind of like routers, they don’t always send messages in a direct route.