Here is a way to restore the buttons "Apply," "Cancel," and "OK" to the Apple Mail 10.3 Preferences panels. With these buttons in place, one does not have to close the preference window or switch to another preference pane to apply changes, and changes can be undone with Cancel.- Make sure Mail isn't running
- Navigate to Mail in the Finder (probably inside the Applications folder)
- Control-click it and select "Show Contents"
- Navigate inside the Contents -> Resources folder
- Open the file PreferencePanels.plist with a text editor. I suggest one should use the Property List Editor (Developer Tools must be installed)
- Change the String Value for "UsesButtons" from 0 to 1 and save the file
- Quit the editor
[robg adds: This worked as described and is much more useful than the simple "OK" button.]

