For land based communications in a post apocalyptic scenario, it is difficult to beat the mail service as pioneered by the ancient Persian Empire, later replicated by the Romans and even in the United States as the "Pony Express". Simply a series of roadhouses at set distances apart so messengers can stop and exchange horses or pass mail bags off the the next person in the chain.
Depending on just "how" post apocalyptic this is, the messengers and even the mail stops may have to be armed and fortified (much like stagecoaches had a shotgun rider, or trains had detectives or armed Pinkerton guards), and of course this does imply that the mailmen are part of a larger bureaucratic structure. Any post apocalyptic warlord who wants to extend their territory might actually consider putting resources into this project since it isn't nearly as resource intensive as invading armies and it buys goodwill among the people receiving postal service, making them much more willing to be allies for other projects.
Crossing oceans in a post apocalyptic setting is actually more difficult. The technology for building and sailing transoceanic vessels will have to be recreated (how many people use charts and sextants anymore when computerized displays and GPS allows accurate navigation by even minimally skilled sailors?), and of course you would be smiling into unknown shores (are the ports and harbours operable on the other side of the ocean? Who owns them now? Do you have to send a lighter ashore to figure out who is there?)
As mentioned there may be remnants of the old systems like ham and shortwave radio still in operation, but the industrial capacity to build and extend that network may be lacking (drawing wire is still needed to make primitive radios, for example).