SMTP server requires a secure connection or the client was not authenticated. The server response was :5.5.1 authentication required

Question

private void button1_Click(object sender, EventArgs e)
{
    SendMaxInvoice();
    try
    {
        MailMessage mail = new MailMessage();
        SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
        mail.From = new MailAddress("fromexample@gmail.com");
        mail.To.Add("toexample@gmail.com");
        mail.Subject = "Test Mail - 1";
        mail.Body = "mail with attachment";

        System.Net.Mail.Attachment attachment;
        attachment = new System.Net.Mail.Attachment(@"C:\Users\User\Desktop\dee.txt");
        mail.Attachments.Add(attachment);

        SmtpServer.Port = 587;
        SmtpServer.Credentials = new System.Net.NetworkCredential("fromexample@gmail.com", "*******");
        SmtpServer.EnableSsl = true;

        SmtpServer.Send(mail);
        MessageBox.Show("mail Send");
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.ToString());
    }
}