1. Take Another Look Before You Send a Message

Don’t send anything you don’t want to send.

2. Do Not Default to "Reply All"

"Reply" is good. "Reply to All" is better. Right?

3. Keep Emails Short

Do not intimidate recipients with too much text.

4. Properly Format Your Email Replies, and Be Lazy

Do you think quoting original text in your email replies perfectly is a lot of work? Don’t let the ‘>’ intimidate you! Here’s a very comfortable, relaxed, quick and still clean and compatible way to reply properly.

5. Write Perfect Subject Lines

Do you make these mistakes in your email subjects? (The key to getting your messages read is not to be clever.)

6. Clean Up Emails Before Forwarding Them

Forwarding emails is a great way of sharing ideas, but make sure the original idea is not hidden in obfuscation.

7. When in Doubt, Send Plain Text Email, Not Rich HTML

Not everybody can receive your fancily formatted emails. Some may even react furious. To be safe rather than sorry, send plain text emails only when in doubt.

8. Don’t Forward Hoaxes

Email hoaxes often contain stories that are intriguing, and sure to irritate. Here’s how to spot and stop urban legends.

9. Use Current Antivirus Software, Keep it Up to Date, Scan for Free

Make sure you’re not spreading worms and viruses via email or act as a vehicle for spreading spam. All this can be caused by malicious emails. Fortunately, there’s protection.

10. Say Why You Think What You Forward Will Interest the Recipient

More and better communication makes better relationships. Here’s a way to spot and share relevant information and foster ties by forwarding emails and links.

11. Do Let People Know Their Mail Has Been Received

Did the spam filter eat my message? Spare others this nagging question and let them know you got their email.

12. Ask Before You Send Huge Attachments

Don’t clog email systems without permission.

13. Talk About One Subject per Email Message Only

Help make the world less confusing. Try to talk about one subject per message only. For another subject, start a new email.

14. Punctuation Matters; in Emails Too

Comma, colon, hyphen and semicolon — all exist for a reason: they make it easier to understand the intended meaning of a sentence. Don’t make life more difficult and possibly less interesting for the recipients of your emails. Pay some — though not too pedantically much — attention to punctuation.

15. Use Acronyms Sparingly

DYK? Not everybody knows every acronym, and they don’t save that much time anyway.

16. Resize Pictures to Handy Proportions Before Inserting Them in Emails

When your photos look good in your email, you look good, too! Here’s how to make sure your images are not larger than screens and mailboxes by resizing them in style — online and for free.

17. Writing in All Caps is Like Shouting

Don’t shout in your emails (and all caps is so difficult to read).

18. Be Careful with Irony in Emails

No, really! I mean it. Honestly!

19. Catch Typos by Printing Your Emails

You can often find typos or misplaced commas neither your spell checker nor you yourself catch when proofreading on the screen.

20. How to Avoid Embarrassing Emails

Avoid embarrassing emails by sending them to yourself only (by default).

21. Set Your System Clock Right

Make sure you don’t send messages from 1981.

22. In Doubt, End Emails with "Thanks"

If you don’t know how to say good-bye at the end of an email, there’s one thing that will almost always be appropriate. Thanks.

23. Where to Put Your Signature

Without a line sub-scripted "sign here", how do you decide where to place your email signature? Look here.

24. Wondering "How to Put That in Writing", Write "That"

Tell it like it is. Have you notices how people who you understand perfectly well when you listen to them become cryptic when they start writing?

25. Why You Should Compress Files Before Sending Them via Email

Smaller is more beautiful, at least when it comes to email attachments. So make files smaller before your send them via email.

26. Avoid "Me Too" Messages

"Me too" is not enough content, but too much annoyance.

Thanks to Heinz Tschabitscher, About.com Guide