You’ve made it to the Fields in Kena: Bridge of Spirits. You have progressed through the level and still haven’t found the one lone Spirit Mail that can be found here. That is because it is hidden beneath the forge. Here is its location.
Spirit Mail location
Progress through the level and defeat the Stone Guardian boss. Once you do, you will be standing at the top of a mountain at the forge. The Spirit Mail is hidden deep within this area but is only accessible now. Head through the forge building and out the other side to the wooden platform that overlooks the Fields. Turn to the left and follow this platform. It will lead you down some stairs and into a cavern beneath the forge. It will look similar to the image below.
Above where you see the Forest tear is a stalactite. When you enter this area, the water will be drained and there will be no way to traverse. You will be confined to the wooden platform. Shoot the Forest Tear that hangs from the stalactite to make it fall onto the blossom.
When you look down below the pressure plate on the right, you will see a group of glowing rocks that you can throw a bomb at. Throw a bomb and use the Forest Tear to create a Rot Cloud. When the bomb explodes, the rocks will float up and bridge the gap between the tear and the pressure plate. Move the Rot Cloud over and have it slam the pressure plate. This will cause the water to rise and fill the room.
Once the room is filled with water, use it to swim over to the opening on the left. There is a wooden platform in the water you will be able to use to jump up. Walk outside and turn to the right to find the Spirit Mail.