尽管我允许使用不太安全的选项，但仍出现“失败的邮件发送”错误

Question

我正在从c＃发送邮件，但它会生成“失败发送邮件”。我启用了安全性较低的选项，但仍然会产生错误。我已经检查了互联网上的帮助，但无法正常工作。请给出适当的解决方案。 下面是我的代码：

private void btnSend_Click(object sender, EventArgs e)
{
    try
    {
        MailMessage mail = new MailMessage();
        SmtpClient smptServer = new SmtpClient("smtp.gmail.com");
        mail.From = new MailAddress("mama.bannu123@gmail.com");
        mail.To.Add(Convert.ToString(this.txtToMailAddress.Text));
        mail.Subject=Convert.ToString(this.txtSubjectMail.Text);
        mail.Body=Convert.ToString(this.txtBodyMail.Text);

        System.Net.Mail.Attachment attachFile = new Attachment("D:\\exp1.pdf");
        mail.Attachments.Add(attachFile);
        smptServer.Port = 587;
        smptServer.Credentials = new 
           System.Net.NetworkCredential("mama.bannu123@gmail.com", "password code");
        smptServer.EnableSsl = true;
        smptServer.Send(mail);
        MessageBox.Show("Message Send Successfully");
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}