As an IC, people rarely read my emails if it doesn't fit in their screen (so 3 paragraphs is the max).
The only thing I've found that works is to send details, and then probe the recipient on each important item in there to make sure they understood what I wrote. If I'm too detailed, they will get lost. If I'm very concise (but still complete), they'll misinterpret.
Getting people to reflect back to you what they understood is encouraged in most communications books. You can write the perfect email, and it will still get misunderstood.
Edit:
It is amusing that people are responding giving me writing advice. Equally amusing is the implicit tone in some of the comments that I'm doing it wrong and implying I don't follow what they are suggesting - especially when I didn't even discuss how I write my emails!
As I said: You can write the perfect email, and there will always be someone who will misinterpret it. The problem is not merely in the content, but in the other (including their worldview, culture, social status, etc). The solution isn't to try to become a more perfect writer, but to understand the dynamics involved in communication (written or verbal).
To paraphrase one of the books: People will interpret what you say based on their worldview - something that you often do not have access to. It's their worldview that will often make them misinterpret something that is very clear and unambiguous in what you say. Accept that reality and work around it.