Here's the secret to getting the highest ROI from email: Write less.
This is easier said than done. It takes hard work as demonstrated in this clip from the great movie, A River Runs Through It:
You'll see the most influential people respond to tons of email with just a handful of words in each.
One method I've learned recently to hone this skill:
Take an idea and describe it in 25 words. Then, do it again in 12 words. Finally, use only 5 words.
My experience is this exercise can hone your ability to write concisely.