There are countless kinds of messages that are best conveyed with an email, but to be the most effective, an email should have one topic. If you need to discuss two major topics, you should be sending two emails. Writing long emails with multiple topics is one of the biggest mistakes most people make.
Regardless of tone or subject, your email should always be checked for grammar. With all of the tools and browser extensions available these days, there's no excuse for anything but pitch-perfect spelling and punctuation. There are also extensions that will help you identify sentences that contain more words than needed to get their subject across. The goal is to say what you want to say without forcing them to read blocks of text.
As a courtesy, always spell things out when referencing events or timelines. This will make it easier for them to search for your email at a later date if necessary.
The last thing to do before sending your email is to take a breath and read the whole thing through. Ask yourself if all of the information is pertinent or if anything might be better off omitted to increase the email's overall clarity and effectiveness. Double-check that the vital information like dates and times are correct and that any referenced documents are attached. Everyone knows the embarrassment of having to send a follow-up email to include the very thing the original email concerned!