As I go through this process, I try not to use email to give someone negative feedback, and I rarely respond to negative feedback over email. Email is a great tool for transactional conversations (Where should we have lunch?), sharing information (Here’s that file, there’s someone I want you to meet), or showing appreciation (You spoke powerfully in that meeting, I’m touched by your support — thank you). For anything else, you’re better off calling or talking to someone face to face. I also do my best never to go back and forth with someone on email about something more than two or three times. If it’s gone that far, it’s usually a better idea to pick up the phone.
Source : HBR