Mate, you're mixing Game of Thrones with a carrier pigeon in a blender. A similar question was asked about two years ago, and while the answers there aren't too much help, I'll tell you that ravens weren't used to deliver messages.

Carrier pigeons, a trained type of homing pigeon, were used to deliver messages. The basic idea is that these pigeons will always return to the nest. You raise them in a certain area, then release them farther and farther away, training them to return over longer and longer distances. Then, when they're ready, you take their cage off to where you need to send a message, put that message in a tube tied to one of the pigeon's legs, and send it home.

The pigeon doesn't return to the spot where you released it -- it's a one-way trip home. To actually exchange messages by pigeon, you'd have to have two one-way sets.

Radiolab had an interesting piece a while back about the homing ability of pigeons (and other animals), and if you're interested in learning about how pigeons and other animals navigate over long distances like this, check out this link.