Yahoo business mail – All of sudden – The server response was: 5.7.1 Authentication required

I am using yahoo business mail as my smtp provider to send email from asp.net site hosted on IIS 7.5
All of sudden users are getting an error. My hosting provider says that it may be issue with yahoo servers. Any other idea what can be wrong? Below is error message and configuration of my web.config

System.Net.Mail.SmtpException
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Authentication required

System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. —> System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Authentication required
at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)
at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from)
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)

<system.net>
       <mailSettings>
     <smtp from="administrator@xxx.com">
                           <network host="98.139.221.242" port="25" enableSsl="false" userName="administrator@xx.com" password="xxx" defaultCredentials="false" />
                     </smtp>
    </mailSettings>
    <defaultProxy>
      <proxy usesystemdefault="False"/>
    </defaultProxy>
  </system.net>

I logged in with above email and password in yahoo and I was able to access the account.