If you get the error "550 Authentication is required for relay" when attempting to send email via Microsoft outlook, outlook express or other email client software. The reason is the email server is configured to require smtp authentication to send out outgoing email.

In order to resolve this issue, you need to enable the smtp authentication on email client. The steps is depends on the version of software that use and also the email client software.

For Microsoft Outlook, please refer below:-

1. Click Tools menu on Outlook
2. Click Email Accounts
3. Click View or change existing e-mail accounts
4. Click Next
5. Highlight an email account
6. Click Change
7. Click More Settings
8. Click Outgoing Server tab
9. Check the My outgoing server (SMTP) requires authentication
10. Ensure that Use same setting as my incoming mail server is selected
11. Click OK, Next then Finish

For Outlook express, please refer below:-

1. Click Tools menu
2. Click 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. Ensure that Use same setting as my incoming mail server is selected
8. Click OK and click Apply
9. Click OK then Close