For an incoming message: Select the message, then choose Save As Text from the Message menu.
For an outgoing message: Choose Text from the Save As submenu of the File menu in a Compose window.
Type a file name and location in the File Selection dialog box that appears, then click Save or Append to either save the message to a new file or append it to an existing file.
If you click Save and a file by that name already exists, you are asked whether you want to overwrite the file or cancel the operation.