This error may indicate that you have neglected to set up smtp authentication in your mail client software.
Please find the smtp authentication setting in your mail client, and ensure that it is set to use the same email address and password you use for incoming mail.
For example, in Microsoft Outlook, open your mail account, click the "More Settings ..." button, then click the "Outgoing Server" tab. Make sure the checkbox to the left of "My outgoing server (SMTP) requires authentication", and that the option to "Use same settings as my incoming mail server" is selected.
See this knowledgebase article for additional email troubleshooting tips.