Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Create your own Mail.app Icon
Many of you will know that it is possible to change the default mail.app icon but did you know creating your own has never been easier ? Well thanks to John Godfrey we can now offer you a Photoshop template that allows you to create your own mail icon in a matter of minutes. Better still you can create your own custom mail style stamps to use in other projects.
The zip file contains instructions on how to insert your desired image into the template. It helps if you rotate it about 11.2 degrees counter-clockwise and resize it to a width of about 85-90 pixels.
The layers are really easy to customise to your own design. If you do want to replace your current mail.app icon then you can use a freeware utility like img2icns to convert it into an icon file.
Once you have your newly created image converted to an icon file then you can add it to your mail.app install. Go to the Mail.app icon (in the Applications folder) and right click. Click on the “Show Package Contents” option.
Within this folder you should see a ‘Resources’ folder you should see the main mail.app icon file. I would take a copy of this file before you add your custom icon, just in case you want to move back in the future.
Tricks & Tips