The golden rule states that you should do unto others as you would have them do unto you. In terms of netiquette, this means that you should not use improper web conduct that you would not appreciate from others. Poor netiquette may include, but is not limited to, annoyances such as an angry tone, utilization of caps, heavy use of the singular first-person pronoun, flaming and spamming.
Your mood is often reflected in your writing, and when you are in a bad mood emails can come across as offensive. The best preventive measure is to read the email before sending it, making sure that the tone is not unpleasant.
It could be that you use all caps just to draw your reader's attention. However, caps are often interpreted as shouting or scolding, which can disrupt harmonious communication.
The First Person
When we speak, it is quite common to do so in the first person, but this can come across as very rude and egotistical in mediums of online communication such as chat or emails. It is better to use inclusive pronouns such as 'we' or 'you' to make the tone a bit more convivial.
Flaming is when you go off on a tirade via web communication without any regard for readers. This is a common activity in forums where people tend to express their emotions without any regard for prudence.
Spamming is the act of sending unsolicited emails, chat announcements or forum posts promoting a product or web page. These unwanted messages tend to fill up email accounts and are the cause of much annoyance on the web.