1. Be concise and to the point
2. Answer all questions, and pre-empt further questions
3. Use proper spelling, grammar & punctuation
4. Make it personal
5. Use templates for frequently used responses
6. Answer swiftly
7. Do not attach unnecessary files
8. Use proper structure & layout
9. Do not overuse the high priority option
10. Do not write in CAPITALS
11. Don't leave out the message thread
12. Add disclaimers to your emails
13. Read the email before you send it
14. Do not overuse Reply to All
15. Mailings > use the bcc: field or do a mail merge
16. Take care with abbreviations and emoticons
17. Be careful with formatting
18. Take care with rich text and HTML messages
19. Do not forward chain letters
20. Do not request delivery and read receipts
21. Do not ask to recall a message.
22. Do not copy a message or attachment without permission
23. Do not use email to discuss confidential information
24. Use a meaningful subject
25. Use active instead of passive
26. Avoid using URGENT and IMPORTANT
27. Avoid long sentences
28. Don't send or forward emails containing libelous, defamatory, offensive, racist or obscene remarks
29. Don't forward virus hoaxes and chain letters
30. Keep your language gender neutral
31. Don't reply to spam
32. Use cc: field sparingly
Of course, I'm guilty of #15 when I got this whole blog and mailing list started, so we all have more learning to do.