If you’re looking for some kind of a checklist that can help you write better email messages and avoid making common mistakes, you might like to try this one here (link to: mindtools.com).
One of my favourite tips is to imagine that you’re talking to the person when writing an email. If it’s an important email, I’ll even read it out loud. Okay, I can do that because I’m alone in my office!. But you can whisper it out loud if you like!