* 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.
Hell, I've learned not to ask more than one question in an email. The first one is the only one to get answered.
I had this recently with something I wanted to order online. I asked two questions, the second was answered, the first was ignored. So I had to send a second email to ask the first question again.
I'm really curious if it's a symptom of limited modern attention spans, or if you'd find the same issue in vintage hand-written letters.