1. If you are sending an email to multiple people, put their email addresses in the BCC field and your own email address in the To: field. No one likes to share their private email address with strangers.
2. The email subject should be detailed enough to give the recipient an idea about the email content without having to open it. Single words like "Hi" or "Hello" or "Help" are a strict no-no. Think of meaningful but short titles.
3. Do not attach large attachments in your email since not everyone is on a broadband connection. If you have to send a large file over email, upload it to services like Yousendit and then pass on the link in the email.
4. If you have to email more than two documents as attachments, zip them in one file. Doing so would ensure that your friend won't miss downloading any file.
5. Do not write an email while you are drunk or in a really bad mood. It would reflect on the style of your writing.
6. Do not request a Read Notification Receipt.
7. Always reply to emails especially the ones specifically addressed to you. The sender is still waiting to hear from you.
8. Keep you email message short and to the point. Sentences like "I hope this email finds you alive and well" look good only in letter correspondence.
9. Do not hit the Send button without doing a spell check. An email with spelling mistakes or grammatical errors indicate that you have written the message in a non-serious mood and may convey a bad impression. Always read the message before broadcasting it to the world.
10. You may want to remove your personal mobile number and the home number from the email signature if you are participating in a big mailing list. You never know the kind of people who have subscribed to that list.