Issue 10 of Email for a reason
Write politely, but naturally. Writing spoken language gives a sloppy impression, while being too formal is perceived as impersonal and contrived.
There is no one perfect level for all occasions, it’s up to you to decide. You will use different levels for different occasions and recipients, just like in spoken communication.
Make it a habit of expressing yourself in a manner that you can stand by in any situation. Would it be OK if your manager, client or family would happen to read it?
No more than five sentences, is a good benchmark för clear emails. Needless to say it’s just a rule of thumb, use your own judgement.
A more absolute rule is: Never email when you are upset. You can write, but don’t send until you’ve calmed down and read it again. In a scenario where you are upset, but are required to respond promptly, be brief, stick with the facts only, and have someone review it before you send.