What is proper email etiquette?

Professional email communication is different than personal email communication. When communicating with clients or instructors, one should use proper professional email etiquette:
  1. use a clear, concise, short subject
  2. address the recipient by name
  3. explain why you are writing the email
  4. keep your note brief and to the point
  5. specify what class you are in (if you are a student)
  6. close with a simple signature (if you are a student, just your name will be fine)
  7. do not format your email (i.e. bold, italics, color, all caps, etc)
  8. do not send attachments more than 1MB (images should be no wider than 1000px)
  9. when replying, include the text of the original note; do not change the subject
  10. when replying, stay on topic (or start a new email with a new subject)
  11. respond within 24 hours (if possible); it's ok to not respond on weekends
  12. use a professional email address (i.e. "sweetpea@hotmail.com" is not a good choice)

Also see Why should I switch to Gmail?