This error indicates that your email client is not configured to use SMTP Authentication. To protect against unauthorized access to our SMTP servers, users are required to authenticate with a valid email account.
See the following articles for information on how to configure your email client to use SMTP Authentication
Outlook Express
Outlook XP/2002/2003
Outlook 2007
Mozilla Thunderbird V.1
Mac Mail
Mac EntourageArticle ID: 536, Created On: 8/10/2009, Modified: 4/21/2011