From the Super Mario Wiki
Mario's Letter is an item of Mario that Luigi finds in Luigi's Mansion. It is found inside a little birdhouse in the Courtyard behind the mansion. This possession, along with Mario's Glove, Mario's Cap, Mario's Star and Mario's Shoe are scattered around the mansion. If Luigi takes them to Madame Clairvoya, she will use her crystal ball to "communicate" with Mario.
When Luigi takes this specific item to Madame Clairvoya, she reads it aloud. After reading, Madame Clairvoya sarcastically comments on how Mario must not be a great novelist.
The letter reads: Look out for Boos, Luigi!