Mailtoad gives letters with the amount of 1-Ups depending on if the player was controlling Mario (5) or Luigi (25). Some of the letters he would have he'd send to the Wii Message Board, though these would never contain 1-Up Mushrooms. He will also send messages from Luigi to come save him (for both Mario and Luigi) and from other characters from the game that will offer the player a Power Star.
Super Mario Galaxy 2
In Super Mario Galaxy 2, he will allow you to view all the mail you have received. There is also more variety in the mail with a host of new characters looking for a challenge.