This comprehensive article from itsecurity.com presents 25 most common mistakes email users make and useful tips on how to prevent them, or as they say "25 tips to bring newbie Internet users up to speed so they stop comprimising your network security." Here is what you should keep in mind:

Properly managing your email accounts
1. Using just one email account.
2. Holding onto spammed-out accounts too long.
3. Not closing the browser after logging out.
4. Forgetting to delete browser cache, history and passwords.
5. Using unsecure email accounts to send and receive sensitive corporate information.
6. Forgetting the telephone option.

Tips for Preventing Common Email Security Issues Emailing the right people
7. Not using the Blind Carbon Copy (BCC) option.
8. Being trigger happy with the "Reply All" button.
9. Spamming as a result of forwarding email.

Making backups and keeping records
10. Failing to back up emails.
11. Mobile access: Presuming a backup exists.
12. Thinking that an erased email is gone forever.

Avoiding fraudulent email
13. Believing you won the lottery … and other scam titles.
14. Not recognizing phishing attacks in email content.
15. Sending personal and financial information via email.
16. Unsubscribing to newsletters you never subscribed to.

Avoiding malware
17. Trusting your friends email.
18. Deleting spam instead of blacklisting it.
19. Disabling the email spam filter.
20. Failing to scan all email attachments.

Keeping hackers at bay
21. Sharing your account information with others.
22. Using simple and easy-to-guess passwords.
23. Failing to encrypt your important emails.
24. Not encrypting your wireless connection.
25. Failing to use digital signatures.

Please find the full article (in original), here:
www.itsecurity.com/features/25-common-email-security-mistakes-022807/