Careerbuilder.com has a great article on Top 10 E-Mail Mistakes. Check out the article for more information. Here is a quick run-down:

1. A vague subject line.
2. No sign-off.
3. Being too casual. (C U l8r!)
4. Going on and on and on.
5. SHOUTING IT OUT.
6. Striking the wrong note.
7. Weighing others down.
8. Sending to the wrong box.
9. Mixing business and pleasure.
10. Over e-mailing.