The reason for this error is that your email server requires SMTP Authentication to send outgoing mail, and is a standard practice amongst most ISP's in order to combat spam.  In order to resolve this issue you will need to configure your email client such as Outlook, Outlook Express, Eudora, or Thunderbird so that you are an authenticated user.

For Outlook do the following:

1)       Click on the Tools menu

2)       Click on Email Accounts

3)       Click View or change existing e-mail accounts then click Next

4)       Highlight an email account and click Change

5)       Click More Settings

6)       Click Outgoing Server tab

7)       Check the My outgoing server (SMTP) requires authentication

8)       Make sure that Use same setting as my incoming mail server is selected

9)       Click OK

10)      Click Next

11)      Click Finish and your done

For Outlook Express do the following:

1)       Click on the Tools menu

2)       Click on Accounts

3)       Highlight an email account and click Properties

4)       Click the Server tab

5)       Under the Outgoing Mail Server, check the “My server requires authentication” check box

6)       Click the Settings button

7)       Make sure that “Use same setting as my incoming mail server” is selected

8)       Click OK

9)       Click Apply

10)      Click OK