Email etiquette refers to principles for proper email communication, such as being respectful, using formal written language instead of slang or sarcasm, and choosing words carefully. It is important for emails to be professional by using a polite greeting, focusing on the topic, and avoiding personal issues. When writing emails, one should use proper spelling, grammar, and punctuation, avoid all capital letters, include a descriptive subject line, and not attach unnecessary files. It is also important to consider who will see the email by being careful of reply-all and CC/BCC functions, and to proofread before sending to avoid mistakes.