Letters as in-game collectibles have made various appearances in the Mario franchise.
Super Mario series
Super Mario Bros. 3
Letters first appeared in Super Mario Bros. 3. Every time Mario clears a world, he will receive a letter from Princess Toadstool, which usually contain a helpful item and an adventure tip. However, the last one is not from Toadstool, but from Bowser, saying he kidnapped the princess. That one was the only one in the NES version to have any musical cue, which had bars from the Boss theme music of the game accompanied by a red flash. In the remakes, however, a calming musical cue can be heard with the letters from Toadstool. In the GBA remake, an extra letter was added into the game for the intro in which it is from Toad, being sent to the princess, where it is addressed to her as "Princess Peach" rather than the older "Toadstool" moniker. However, the princess's letters still retain the Toadstool moniker as opposed to using the original Japanese and the current universal "Peach" name. In the 25th Anniversary version of Super Mario All Stars, her sending letters was referenced with a letter sent by Nintendo with Peach as the in-universe author of the letter.
Super Mario 64
A letter appears in Super Mario 64. Mario receives one from Peach at the very beginning of the game, telling him to come to the castle to meet her, but as soon as he arrives, he finds that Bowser has stolen the Power Stars and kidnapped Princess Peach.
Super Mario Galaxy
At the beginning of Super Mario Galaxy, Princess Peach sends Mario a invitation to the Star Festival. Later in the game, letters are sent to Mailtoad, who then notifies Mario or Luigi when they arrive. In Super Mario Galaxy, letters are primarily sent from Luigi when he has become stuck in a certain galaxy and cannot get back without the player's help. The player will occasionally receive letters sent from Princess Peach, which express her concern at being kidnapped (though she is sure the player will rescue her), and also contain 5 or 20 1-Up Mushrooms.
Super Mario Galaxy 2
Super Mario Galaxy 2 also starts with Princess Peach sending Mario a letter to watch the Star Bits fall while they eat cake together. Later, letters are sent from a much wider range of characters than the first game, such as Gearmos and Jibberjays. They are used to accomplish a number of objectives, including notifying the player of any of The Chimp's challenges, giving the player tips on how to perform certain moves more expertly, advising the player of a special Power Star opportunity in a particular galaxy, offering the player advice and encouragement, challenging the player to a race, or making the player aware of any other special events or occurrences that may arise periodically as the game progresses.
Super Mario 3D Land
The beginning of Super Mario 3D Land has Bowser sending a letter stating that the latter had kidnapped Princess Peach, thus serving as the main reason for the game. Between levels, Peach sends letters of her own, which reveal that she made efforts to escape from Bowser and his forces, although by the penultimate level, Bowser had her recaptured, and sent a letter taunting Mario about how he has her (also placing her cage near his throne to ensure she cannot escape his grasp). After the ending, a letter is shown of Mario sitting with Peach on the Tail Tree. In addition, another letter is sent by Luigi requesting that Mario rescue him. If the player beats the game as Luigi, the letter shown will be the same as before, only Luigi replaces Mario as who Peach is sitting with. After completing the game fully, a letter is shown depicting Peach accidentally getting a Tanooki dress.
Unlike most instances of letters in the Mario franchise, there is no actual text given for the letters, just a photograph and various sound effects that imply the contents of the letter.
In Hotel Mario, the intro has Bowser sending Mario and Luigi a letter assuming that he and the Koopalings have taken over the Mushroom Kingdom, leading Mario reading it for Luigi. As read aloud by Mario in the cutscene, the letter reads as follows:
Paper Mario series
Later on, Letters appear for the first time as items. Parakarry dropped them all over the Mushroom Kingdom during delivery. In Chapter 2, he requests that Mario help him find the three letters he has dropped on Mt. Rugged. After Mario finds them all and gives them to Parakarry, Parakarry joins his party. Mario can find other letters Parakarry has dropped during his adventure. Delivering them to whoever the letter is written to usually rewards Mario with a Star Piece.
A large chain letter sequence appears as a long side quest. A letter at the end of Mt. Rugged starts the whole thing which will last until Chapter 7. At the end, the player will get the Lucky Day Badge. The player must have Parakarry out when giving a letter.
Mario and his partners also get letters from people all throughout the game, which can be checked at Mario's house and the Toad Town Post Office. Mario receives ten from the various characters he meets.
In the ending, Mario and Luigi, alongside the various party members, are given letters inviting them to the celebrations regarding Bowser's defeat and the retrieval of the Star Rod.
Paper Mario: The Thousand-Year Door
In this game, Mario no longer receives letters from people he meets. Instead, he receives e-mails on his Mailbox SP. He does, however, receive a letter prior to the start of the game from Princess Peach via Parakarry, with Peach requesting that Mario head over to Rogueport regarding something she discovered.
Before Chapter 5 of Paper Mario: The Thousand-Year Door, Podley has a letter for Admiral Bobbery written by Bobbery's deceased wife, Scarlette, who wrote it as she was lying on her deathbed. Delivering the letter to Bobbery strengthens his resolve and he agrees to accompany Mario to Keelhaul Key.
Paper Mario: Sticker Star
Although not an actual item in Paper Mario: Sticker Star, a certain part of Shy Guy Jungle has several ounces of junk documents present that Mario has to sift through. Some of them were letters, including one from Goombella titled "Observations on the Ancient Civilization of the Chomp Ruins."
Paper Mario: Color Splash
In Paper Mario: Color Splash, Mario finds a letter in a treasure chest on Fortune Island from the Snifit or Whiffit crew informing him that they had taken the Instant Camera Thing for a prize for their show.
Although not a true letter, Peach does hand him an envelope that had been sent to her by someone that had been made out of the drained remains of a hapless Toad, which resulted in her and Mario heading over to Prism Island to investigate due to the postage stamp indicating where it was sent from in the beginning of the game.
A letter also appears in Luigi's Mansion, which is one of Mario's missing items. It reads, "L-o-o-k--o-u-t--f-o-r--B-o-o-s,--L-u-i-g-i-!".
Super Mario Run
Super Mario Bros 3
Super Mario All-Stars version
Super Mario Galaxy
Super Mario Galaxy 2
Super Mario 3D Land
Names in other languages