To me it shows a lack of respect or compassion if you do it by email, email to me represents something unpersonal and low effort. Writing a letter is a different thing. Face to face is apropriate in my book, as it means you face the other person and let yourself be exposed to the immediate disappointment of the other party. Cannot really explain why, that's just the way I feel.
There are these stories about dumping the GF by SMS, it's kinda the same thing as email to me.
