Communication experts say most of what we hear, really hear, is communicated non-verbally. For example, only 7% of what we hear is words, 55% of what we hear comes through body language, and 38% comes through the tone of voice.
This is why email is such a tricky tool of communication. You cannot see people's faces or hear their tone of voice. It is so easy to misunderstand what people are saying or what they mean when all you have is black or white. A light-hearted comment can be taken as a critical remark, or vice versa.
When the communication is important, skip the email and look the person in the eye.