How Do I Fix Sending Email Problem In C# ?

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C#

ASP

SMTP

Hi All,
i trying to simple develop website and i need to add sending email in it

i try many format of codes, but still i get same problem
the problem that I face says:
in Chrome browser : The remote certificate is invalid according to the validation procedure.
in Firefox & Internet Explorer : Failure sending mail.

This is the code ,, and i saw that in youtube and it is working with the person who write it ,, but I don't know what is the problem with my project ..

The Code :

MailMessage msg = new MailMessage();
            MailAddress to = new MailAddress(Label24.Text.ToString(), " Trainig Management System");
            MailAddress from = new MailAddress(Label15.Text.ToString());
            msg.To.Add(to);
            msg.From = from;
            msg.Subject = Label20.Text.ToString();
            msg.Body = Label23.Text.ToString();
            msg.IsBodyHtml = true;
            msg.SubjectEncoding = System.Text.Encoding.UTF8;

            // to read mail information smtp simple transfer protocol
            SmtpClient client = new SmtpClient();
            client.Host = "smtp.gmail.com";   // google smtp setting
            client.Port = 587;

            // to get the 'from' address details ( user name + password) 
            System.Net.NetworkCredential myaccount = new NetworkCredential(Label15.Text.ToString(), TextBox1.Text.ToString());
            client.Credentials = myaccount;
            client.EnableSsl = true;  // encription, make it secure (the sender infromation) 
            client.UseDefaultCredentials = false;


            try
            {
                client.Send(msg);
                Server.Transfer("end_message.aspx", true);
            }
            catch (Exception ex)
            {
                Label19.Text = ex.Message;
            }