Problem:

Why do I receive error "503 valid RCPT command must precede DATA" when attempting to send email?

Solution:

This usually is a result of the mail server requiring authentication.

If you are using Outlook 2003, here are the steps to enable authentication:

Open Outlook

1. Go to Tools | Email Accounts
2. "View or change existing e-mail accounts" should already be marked
3. Click Next
4. Select the Account with the trouble and click Change
5. Click on More Settings
6. Click on the Outgoing Server Tab
7. Click "My outgoing server (SMTP) requires authentication" and make sure "Use same settings as my incoming mail server" is selected
8. Click on the Advanced Tab
9. Set "Outgoing server (SMTP):" port to 290 And Enable SSL 
10. Click OK and then Test Account Settings
11. If everything is fine and reports no errors,
    • Click Close.
    • Click Next.
    • Click Finish.
12. If an error does occur, it will switch to the Errors tab and display the error

Note: If you are still having errors, make sure your login username is your full email address and that the password is correct.