The current version requires OS X 10.11 El Capitan.
Upgrade to OS X El Capitan for free.
Visit the OS X El Capitan page on the Mac App Store. Click the download button, and follow the onscreen instructions to begin your upgrade.
Version: 4.0.4 released on: Jan 13, 2016
Compatible: OS X El Capitan (Mail 9.3)
Double-click the DockStar Installer in Downloads
Follow the Installers instructions
Open OS X Mail application
You're All Set