Your message did not reach some or all of the intended recipients.
The following recipient(s) could not be reached:
550 5.7.1 Unable to relay for email@example.com
Solution: This was the Solution which helped me to send the messages all over the world.
1) Open OutlookExpress2003
2)Go to Tools- Email Account- change email- double click ur mail account
3)Click MoreSettings- Select Outgoing Server tab Check the check box My Outgoing Server SMTP Requires authentication
4)Select Logon on using and give your userID and Password
5)Click Ok and Next and Finish.