From Bulbapedia, the community-driven Pokémon encyclopedia.
Entering a Wonder Mail Password in Red Rescue Team
Wonder Mail is a feature in the Pokémon Mystery Dungeon games. It provides the player with missions to complete in order to receive rewards.
Each in-game mission has its own 24-character password, which is visible in the mission description. These codes can be written down and later entered in the Wonder Mail option on the main menu to put that mission in the player's job list. Special Wonder Mail codes are periodically available on Pokémon.com, which give access to unique missions or dungeons. Wonder Mail is ranked by difficulty, with the lowest difficulty being E, and the hardest being * (in Blue and Red rescue team), or ☆9 (in Explorers of Time, Darkness, and Sky). A small selection of Wonder Mail can also be found on Nintendo's Wi-Fi Connection.
Players can also be rescued by using Wonder Mail codes to activate the rescues, and the A-OK and Thank You message can also be sent with a Wonder Mail code.
In Explorers of Sky, Wonder Mail has 34 digits, and is renamed Wonder Mail S. Wonder Mail from its sister games Time and Darkness is unusable.
In Gates to Infinity, it is once again named Wonder Mail, but now only has eight characters. A sample code is shown in the Manual on the 3DS, which is "XN83N4W6", which gives the player three Reviver Seeds.
It is possible to use a Wonder Mail Generator to create a code for a mission for any reward, even one that could never happen in-game. For example, one could generate a code to rescue a Pokémon in Tiny Woods, the easiest dungeon, and receive the very valuable Friend Bow as a reward. One could also create a code to rescue someone in an easy dungeon like Beach Cave, with the reward being something one can only get in a locked door in a specific difficult dungeon, like Moltres's Fire Veil, thus bypassing the effort and challenge needed to get the item normally. Furthermore, the effort can be bypassed farther by setting it to "escort" for the first floor, thus allowing you to automatically achieve the goal the moment you enter the dungeon, and avoiding even battling any Pokémon. Some people use it as an opportunity to gain easy rescue points, and have it set to a 1-floor mission in a difficult dungeon, such as Purity Forest (a mission that could be ranked up to S difficulty). Many also use Wonder Mail generators as a way to recruit hard to get Pokémon with ease by picking a rare Pokémon as the client and having it join the team as reward.
Like Pokésav, the legitimacy of using these generators is highly questioned. If one such generator is used to create a mission in a dungeon required for the main story, but not yet unlocked, the game will say: "That's odd. The location of this mission is dungeon. It seems to be outdated." The same message would appear for the dungeons that can only be unlocked through missions, but already unlocked.