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

try
{
    MailMessage Msg = new MailMessage();
    // Sender e-mail address.
    Msg.From = new MailAddress(txtemail.Text);
    // Recipient e-mail address.
    Msg.To.Add("manojmuraliconnect@gmail.com");
    Msg.Subject = "Enquiry";
    Msg.Body = "Sender : " + txtemail.Text + "\n" + "Name : " + txtname.Text + "\n" + "Phone : " + txtphone.Text + "\n" + "Address:" + txtaddress.Text + "\n" + "Enquiry:" + txtenquiry.Text;
    // your remote SMTP server IP.
    SmtpClient smtp = new SmtpClient();
    smtp.Host = "smtp.gmail.com";
    smtp.Port = 587;
    smtp.Credentials = new System.Net.NetworkCredential("manojmuraliconnect@gmail.com", "Password");
    smtp.EnableSsl = true;
    smtp.Send(Msg);
    ScriptManager.RegisterStartupScript(this.Page, this.GetType(), "ASET", "<script type='text/javascript'>alert('Your Message Has Been Send ');</script>", false);
    CleaContents();
}
catch (Exception ex)
{
    lbltext.Text = ex.Message;
}