I've created a replacement Mac OS X Mail icon so it feels more British rather than American. The Robin has recently been voted the national bird by than 224,000 people, so it felt apt to use the Robin instead of the Eagle.
To replace the Mail icon in Mac OS X 10.11 El Capitan you need to do the following:
- Restart your Mac in Recovery Mode by holding CMD+R at startup
- Go to Utilities > Terminal in the menu
- Type 'csrutil disable' and Enter
- Restart normally and change your Icon by showing the 'Get Info' windows and copying and pasting the icon.
To re-enable System Integrity Protection (SIP or Rootless)
enter csrutil enable by using the same method.
Download the .icns file from here