"I would have written a shorter letter, but I did not have the time."
~ Blaise Pascal
What does the reader need to know and do?
Keep each email to a single idea or request. Each paragraph is a single idea. Each sentence is related to a single idea. Write painfully "choppy copy".
Aim for 50-125 words.
When editing always revert back to Thomas Jefferson's advice,
"Never use two words when one will do"
Write your email, then write out the bottom line or the TL;DR (Too Long; Didn’t Read).
Only send the TL;DR.
When in doubt:
"Be Brilliant. Be Brief. Be Gone."