Error when sending mail ,Works fine on my machine, error is only on host

hi,
when i am trying to send a mail,an exception is caught :"The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at"
this is my c# code..
SmtpClient client = new SmtpClient(smtp.gmail.com,587Go to the complete details ...