1. Don't send an email as a knee-jerk reaction.
Do not hit 'send' in a vulnerable, emotional state of mind especially when you're angry, hungry or tired.
2. Don't ramble
Put the most important information at the top of the email.
3. Don't conduct personal business
Company time and property should only be used for company work.
4. Don't gossip (about people)
Gossip is unprofessional, and may come back to bite you. You are not able to control how an email gets circulated after you have sent it.
5. Don't write derogatory stuff.
Don't write anything derogatory against a person's religion, ethnicity, gender or sexual orientation - these things hurt, and can result in lawsuits or other EEOC issues. Besides, sarcasm jokes do not travel via email.
6. Don't criticize or call out other people.
Basically it means, don't be personal. Don't say things which you won't in a face to face meeting. If you have to criticize, state reasons in a factual and reasonable way. Focus on the issue at hand, not the person.