Postage is paid at the port of exit so what benefit is there for the destination location to deliver any letter for which they receive no financial benefit.
Similarly let's say I have a letter transitting through another country (Toronto -> New York -> Johannesburg) so what benefit is there for the transit location?
Why would one country agree to deliver mail from another country? Is there a memorandum of understanding between each country?
I can understand if it was something like FedEx or UPS because they have a worldwide presence and handle the delivery from beginning to end; however
Canada Post or USPS don't continue with the letter to the very end.
So how exactly did the whole world end up agreeing to delivering each other's mail?