You can change icon (and description) to anything you like. At least with nova launcher. I used icon from " Darkful Icon Pack - Theme for Apex/Nova Launcher" (from playstore). Its have tons and tons of different various icons for all sorts of needs. Highly recommended.
This is how it looks on my phone:
p.s. And of course I did not update to latest version of K-9 mail becouse important/must have feature removed.