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. Employ capital letters sparingly.
Some people view it as a sign of distrust.
It is okay to use it if you have reason to question whether the person will log on to receive your message. Check your mail regularly and, whenever possible, respond to people who write to you.