* Use a few bullet points to put attention on the main points you want to convey.
* Without going overboard, use a tasteful amount of graphic design (bolding one key sentence or whatever).
* Break up a giant nuanced email into sections.
* If something is critical, make it visual: a picture, explainer video, or an infographic can be really useful for something key.
This is harder than it looks. A quote attributed to Mark Twain is "I didn't have time to write a short letter, so I wrote a long one instead." It's a lot easier to go overboard than to distill what needs to be conveyed into the core elements.