Have you ever forwarded an e-mail without the permission of the person who sent it to you? If so, you may have violated e-mail etiquette:
"If somebody sends you information or ideas by email, you should not assume that you have their permission to reproduce that information in a public forum (discussion group, USENET newsgroup, chat site etc.) Email is one-to-one for a reason: it is designed for personal communication. Unless you are explicitly told otherwise, always assume that email you receive has a big 'PRIVATE' stamp on it -- so don't spread it around! Even simply forwarding an email to a friend could under certain circumstances be considered a breach of trust by the original sender."