9 Most important characters of business emails in English

1. Clarity

- The content should be simple, easily to follow, short words rather than long one.
- Use the active voice, positive words rather than complaint one.

 2. The "One Thing" rule

- Keep the message focused. The another thing should be written in another email.

3. Be political

- The message is in more the two paragraphs, it should be reduced.
- Use "Please" and "thank you"
- Use phrase that states you are not sure of something: I think that..., It is possible..., The optimal solution...

4. Meaning Subject

Bad: Important. Please read
Better: Meeting set for tomorrow at 10AM - Need a room

5. Think before you write

- Don't send e-mails in haste
- Avoid using context (background information)

6. Understand you Audience

- What they like to hear
- See your writing from their perspective

7. Sign-off like a professional

- Add signature block with appropriate contact information

8. Make sure no errors with spelling and grammar before you click the "Send"

9. Begin and end with appropriate words

- Beginning of an email: rather using "statements" than "question". For example: "How was your weekend?" -> "I hope you enjoy the weekend!"
- End of an email:
  + "Take care", "Cheers": informal
  + "Best", "Regards": formal
  + "Thank you": all cases. :)

References:
[1]. http://www.slideshare.net/jvuu/how-to-write-bulletproof-emails-in-english