The key to all communication, particularly written communication is simple: Bottom Line Up Front (BLUF).
We all get a lot of emails - too many emails. In addition, most of the emails are too long. Put those two issues together and you have the perfect recipe for bad communication.
What I work hard to do is put the thesis statement somewhere in the first 3 sentences. Anything longer than that and people won't read it.
The pushback I get from people is that they can't possibly communicate what they need to in the first three sentences. They miss the point. BLUF isn't to communicate everything, it is to communicate the key information that lets people decide to read. In addition, having to do this makes me think long and hard about what message I am trying to communicate.
For example, my first sentence might be "Below is the information that compares and contrasts the three software systems we are considering". Simple - clear - let's the reader move on or decide to focus on it later.
Contrast that with this: "In our last meeting, we discussed several different software options and had great discussions on the features of each. While the meeting notes did a nice job of summarizing the main points, it was clear to me that we needed a more concise and complete description of the 3 systems so that we could make an honest and effective evaluation."
Which of those two is faster and lets the reader know what the email will be about?
For gosh sakes, no one has time to read your long email. Keep it brief and remember BLUF.