You should treat messages as though you are next door and could have a face to face conversation with the person you send it to. You should use correct writing and not write lengthy emails that get off point. Never post anything when you are upset. If you are unsure of what you are posting ask a trusted person to review it first.
Agree! If you would not say it to their face or out loud in the middle of the cafeteria, then don't say it online. (My mom used to tell me that about writing notes back and forth, back in the old days!)