How to Change the Mail Dock Icon in OS X 10.6 to 10.10
You could change the icons for built-in apps way more easily in versions of Mac OS X preceding El Capitan (10.11). Here’s how to do it if you haven’t upgraded yet.
How to Change the Mail Icon in Mac OS X El Capitan and Later
Newer versions of Mac OS X and macOS have a protocol in place to prevent you changing the icons on the included apps. It’s a security feature, but it will stop you from quickly changing the Mail icon. Follow these steps to make it possible to change system icons.
How to Restore the Default Mac OS X Mail Dock Icon (OS X 10.6 to 10.10)
To bring back the default hawk icon to Mail:
If Your Icon File Has No Preview
To convert a PNG, TIFF, GIF or JPEG image to the proper .icns format, you can use the iConvert Icons online conversion tool. If you do have a .icns file but it still lacks the preview necessary to copy the icon to Mail, you can create it with Image2icon.
