That's good advice in general.
I know that otherwise well-reasoned emails seem like a rant once they reach a certain length. I kept the body of the email to 500 words, and included a pdf of supporting documentation. I let the email sit for a week before I sent it, and had another manager read it as well to make sure the tone was alright.
My entire argument was contained in the body of the email. I knew that was all it would take to get my boss on my side.
The PDF was aimed at HR. The IT director has a lot of power under the IT policy, and he has used it in a retaliatory manner in the past. My goal was to stop the IT director from retaliating so I could do my job while I look for a new one.
I write my emails as if they could be leaked to the general public at any time, and I'm certain that the email wouldn't reflect poorly on me, even out of context. Maybe there was a better course of action, but I don't think I did much to hurt myself.