ASP.Net GMAIL: Mail sending works in local but not on server

hi

refer below link

http://aspsnippets.com/Articles/Send-user-Confirmation-email-after-Registration-with-Activation-Link-in-ASPNet.aspx

I used above code to send Confirmation-email but below error happen

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

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.  Exception Details: System.Net.Mail.SmtpException: 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 Source Error: 

Line 87:         smtp.Credentials = NetworkCred;
Line 88:         smtp.Port = 587;
Line 89:         smtp.Send(mm);
Line 90:     }
Line 91: }

best regards

Neda