How can I replace the Mac Mail icon in Mac OS X 10.11 El Capitan? I'm going to give Mac Mail another try. But I can't stand the stamp icon. I'd like to replace it with the iOS icon.

I have the .icns file but the old "get info" trick where you just paste the icon over the existing one doesn't work.  I've also tried "Show Package Contents" on the Mail.app and replacing the ApplicationIcon.icns from the Finder and Terminal under sudo. No luck.

How can I replace the Mac Mail icon with something a little more consistent with the rest of the OS?
The proper way is to:


boot your Mac to Recovery Mode by holding CMD+R at startup
Go to Utilies and Terminal in the menu
Type csrutil disable and Enter
Restart normally and change your Icon


Repeat the procedure to re-enable System Integrity Protection (SIP or Rootless) but this time, enter csrutil enable

that's it !