IMO he left out: Consider your audience.

These rules should differ depending upon who you are emailing, and matter most when you don't have a strong relationship with the person. One great thing about email is that when two people are familiar with each other, or in the midst of working on something, they can fire information back and forth without having to provide a context or a conclusion.

Email serves multiple purposes. It can work well as a telegraph, or a thoughtful letter. The important thing is knowing which you are composing and why.