您应该使应用程序能够访问Gmail帐户。 此链接将帮助您

below code is working fine in local, but if upload upload it on godaddy it is not working.

Error: showing

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

Line 53:        
Line 54:         smtp.EnableSsl = true;
Line 55:         smtp.Send(message);
Line 56: 
Line 57: 

plz check below code

    MailMessage message = new MailMessage();
    message.From = new MailAddress(emailid.Text);

    message.To.Add(new MailAddress("receiver@gmail.com"));

    message.Subject = "NaatKhawan's Feedback";

    message.Body = "Selected: " + DropDownList1.Text.ToString() + "<br/><br/>Name: " + name.Text.ToString() + " <br/><br/>Email: " + emailid.Text.ToString() + " <br/><br/>Contact Number: " + phone.Text.ToString() + " <br/><br/>Message:<br/> " + remabox.Text.ToString();

    message.IsBodyHtml = true;

    // finaly send the email:
    SmtpClient smtp = new SmtpClient();
    smtp.Host = "smtp.gmail.com";

    smtp.Port = 587;
    smtp.Credentials = new System.Net.NetworkCredential("123@gmail.com", "123");

    smtp.EnableSsl = true;
    smtp.Send(message);
    lbltxt.Visible = true;
    name.Text = "";
    emailid.Text = "";
    phone.Text = "";
    remabox.Text = "";

    lbltxt.Text = "Thank you for supporting NaatKhawan";

you should enable application to access gmail account. This link will help you