Message Bottles (known as Note in a Bottle in Animal Crossing: Wild World) are items that occassionally wash up on the beach. They are glass bottles which contain a letter and may sometimes contain a present.
In Animal Crossing: Wild World and Animal Crossing: City Folk
Notes in Bottles can only be found in Animal Crossing: Wild World, but can be obtained in Animal Crossing: City Folk if a player moved from their Wild World town and had one in their catalog. They will all say something different. The player can also buy bottles in Tom Nook's store for 200 bells each and, unlike most items, the store will have unlimited stock. If the player writes something inside a bottle and releases it and a friend has done the same, when the two connect through Contact Mode/Tag Mode, they will each find their friend's Note-in-a-bottle washed up on their beach. They can be saved in the Town Hall like letters. Presents can also be sent along with the notes, but it is unknown if they reach the other end. The player may also find various random notes in a bottle on the beach with a random message enclosed.
Mail differs depending on whether it is from a real person or Nintendo, as a note from a real player will always start with "Dear Some Stranger," unlike a Nintendo note-in-a-bottle.
In Animal Crossing: New Horizons
Message Bottle in New Horizons.