It's easy to accidentally send a message to the wrong person. It could be embarrassing. Don't send attached files unless you have a very good reason and only if the person is expecting them. They take a long time to download which can inconvenience the person who receives them. Don't be a spammer.
Personal messages are almost always OK, but many people object to electronic junk mail.
Stay on the topic if you're posting to a public bulletin board, forum or news group. Most are focused on a specific topic, and messages that stray too far from the topic can be annoying. Briefly describe who you are if the recipient doesn't already know. Be aware that e-mail can be archived and, under certain circumstances, may not be secure. All archived e-mail can be subpoenaed and made public via a court order.