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All the following points lead up to the most important rule, do not write a message when you are angry or upset.
With all forms of digital communication it is important to keep in mind that you are actually communicating with other people. This is sometimes easy to forget when you are sitting alone in front of your own computer.
Remember that a written text does not convey your feelings in the same way as the spoken word. Other people might interpret the message differently than you. And vice versa, remember that your interpretation of a message that you have recieved might not be the same as the senders. Irony is very often lost in text.
Keep in mind that everything you write (e-mails, but also things you post on the internet and in newsgroups), will be stored on different servers and computers and you will probably be confronted with it when it is least convenient.
Hence, never write or post anything you cannot validate later.
Always be critical to information you recieve.
When in doubt, try to verify information from mulitple independent sources.