Mail Sending problem

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SQL-Server-2005

ASP.NET

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hi
i create a mail code like

MailMessage loginInfo = new MailMessage();
loginInfo.To.Add(txt_email.Text.ToString());
loginInfo.From = new MailAddress("my email id");
loginInfo.Subject = "Contact Detail";
loginInfo.Body = "Message: " + txt_message.Text; 
loginInfo.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.EnableSsl = true;
smtp.Credentials = new System.Net.NetworkCredential("my user name", "My password");
smtp.Send(loginInfo);

and it's work properly in localhost but i host this application and send mail when error are come like this

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 32:         smtp.EnableSsl = true;
Line 33:         smtp.Credentials = new System.Net.NetworkCredential("my id", "my pwd");
Line 34:         smtp.Send(loginInfo);
Line 35:     }
Line 36: }