Hello From Blumenau!

Mail app replacement icon with photo from a place called "Vila Germânica" from Blumenau/SC (Brazil).

The proper way to change the macOS Mail app icon is:

1. Boot your Mac to Recovery Mode by holding CMD+R at startup
2. Go to Utilies and Terminal in the menu
3. Type "csrutil disable" and [ENTER]
4. Restart normally and change the Mail app icon using CandyBar or just dragging the icon on Mail app info window (CMD + i)
5. Repeat the procedure to re-enable system integrity protection (SIP) but this time, type "csrutil enable"

That's it!

Credits to Louie http://louie.land/icons