El Capitan won't save change in default mail reader In El Capitan, whenever I try to change my default mail program from mail.app to Spark, it reverts my preferences as soon as I close the preferences pane.
If the program you wish to select as the new default is in the dropdown menu of available mail programs, there is some Apple bug that won't save it.  However, if you click the "select" option and select your preferred mail reader program from your Applications folder, it will save the new default mail application.