How does international mail/post work?

You can send a letter from say, the middle of the Amazon rainforest to the Gobi desert, and be reasonably confident it will get there. How on earth can this be possible? I know air mail plays a big part, but won't the letter pass through many countries? You pay the cost of the stamps to the postal service in the country you send the letter from. Why do other postal services bother forwarding the mail? What do they get out of it? Are there agreements between all the postal services in the world to forward mail and get nothing in return?



I'm asking a pretty complex question here and probaly not putting it across very well, but it's something I've always been curious about so I hope somebody understands what I'm trying to ask.How does international mail/post work?
Yeah, well, you're never guaranteed that it's delivered. Depending on the country or region, you may be more or less confident that things will get there. Friends in some countries tell me never to mail anything of value or it will get "lost" in their country, and they also say that mail often gets opened and read before it's delivered.



Mostly, though, things DO get delivered in most places, and it kinda operates on the assumption that "you deliver mine, and I'll deliver yours". Rarely will it pass through many countries, though. Normally, it goes by air directly to the other country, and then gets put into their system, so basically it only travels in 2 countries (unless its a very isolated place without direct airflights).