Relevance. Email etiquette and professionalism are both relevant and here to stay for the foreseeable future. It is one of the dominant ways businesses document and communicate their messages.

Email Frequency. Some research estimates the average employee will spend more than ¼ of their work life writing & responding to emails. That totals up to 2hrs/day, 10hrs per week, more than 500 hours year.
Structure. Every email should have a basic form that has certain components. These include: the subject line, greeting, body, and closing.

Don't forget. Grammar, tone, and conciseness are key to a good email.