I wrote the following code to send an email

private void btnMail_Click(object sender, EventArgs e)
{
    try
    {
        MailMessage Msg = new MailMessage();
        Msg.From = new MailAddress("sender@gmail.com");
        Msg.To.Add(reciver@gmail.com);
        Msg.Subject = "One Time Password";
        Msg.Body = "Your One Time Password is: " + txtOTC.Text.Trim();
        Msg.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.Port = 587;
        smtp.Credentials = new System.Net.NetworkCredential("sender@gmail.com", "lab");
        smtp.EnableSsl = true;
        smtp.Send(Msg);
        MessageBox.Show(("Your OTP sent Successfully..."), "Thank you", MessageBoxButtons.OK, MessageBoxIcon.Information);
        //txtEmail.Clear();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
}

I have entered the correct login details but it showing me an error. hereby screenshot is attached.

how to fix this issue?