[solved]The SMTP server requires a secure connection or the client was not authenticated. The server response was: authentication required

If you have not configured credentials properly like this error will occur. Your must need to include Credentials for secure connection then only it will be authenticated.

var client = new SmtpClient("yourserver", 80)

client.Credentials = new System.Net.NetworkCredential(adminEmail, adminPassword);

 you can also check these links hope these are help you...

http://www.infinetsoft.com/Post/How-to-configure-godaddy-email-account-using-net/56