In Mail-It 3.0, you can replace the default mail folder icons with custom icons from any image. Right-click a folder and access its properties, then drop any image into the small blank box at the upper right. The 16×16 icon for that image will be used instead of the folder icon.
To remove the custom icon, select the small box in Properties and press Delete (Alt+X won’t work, as it does in FileTypes panels).