In the Pokémon Mystery Dungeon™: Rescue Team DX game, Wonder Mail allows you to enter eight-character passwords to receive things such as items that will be useful on your adventure. Some of those passwords will get you not just items but also requests to head out and rescue Pokémon, which may allow you to recruit special Pokémon to your team if you complete them.

For a list of Wonder Mail passwords, please check the game’s official site.