sending mail through windows application problem

Question

I have a windows application and i have a smtp exception which states: "The SMTP server requires a secure Connection or the client was not authenticated. The server response was:5.7.0. Must issue a STARTTLS command first. z8sm5630522wfj.13

at System.Net.Mail.SendmailAsyncResult.End(IAsyncResult result)
at System.Net.Mail.SmtpTransport.EndSendMail(IAsyncResult result)
at System.Net.Mail.SmtpClient.SendMailCallback(IAsyncResult result)"

Please help in this regard. I need it urgently.

Thanks in advance,

Avinash