Google recaptcha v2 not working consistently

protected static string ReCaptcha_Key = "<key>";
protected static string ReCaptcha_Secret = "<key>";

public bool IsReCaptchValid()
{
    var result = false;
    var captchaResponse = Request.Form["g-recaptcha-response"];
    var secretKey = ReCaptcha_Secret;
    var apiUrl = "https://www.google.com/recaptcha/api/siteverify?secret={0}&response={1}";
    var requestUri = string.Format(apiUrl, secretKey, captchaResponse);
    var request = (HttpWebRequest)WebRequest.Create(requestUri);

    using (WebResponse response = request.GetResponse())
    {
        using (StreamReader stream = new StreamReader(response.GetResponseStream()))
        {
            JObject jResponse = JObject.Parse(stream.ReadToEnd());
            var isSuccess = jResponse.Value<bool>("success");
            result = (isSuccess) ? true : false;
        }
    }
    return result;
}

protected void btnSend_Click(object sender, EventArgs e)
{
    string Name = txtName.Text;
    string EmailAddress = txtEmail.Text;
    string phone = "Number not supplied";
    if(txtPhone.Text != "")
    {
        phone = txtPhone.Text;
    }
    
    string SendAddress = @"<email address>";
    string Subject = "Message from " + Name + " Top Tree Fellas Website";
    string Message = Regex.Replace(txtMsg.Text, @"\r\n?|\n", "<br />");
    if (String.IsNullOrEmpty(Name) || String.IsNullOrEmpty(EmailAddress) || String.IsNullOrEmpty(Message) || !(IsReCaptchValid()))
    {
        lblError.Text = "Please ensure all required fields are filled in.";
    }
    else
    {
        try
        {
            MailMessage msg = new MailMessage();
            msg.To.Add(new MailAddress(@"<email address>"));
            msg.Subject = Subject;
            msg.From = new MailAddress(SendAddress);
            msg.IsBodyHtml = true;
            msg.Body = "<p>Contact Name: " + Name + "<br />";
            msg.Body += "Email Address: " + EmailAddress + "<br />";
            msg.Body += "Phone: " + phone + "<br />";
            msg.Body += "Message:<br />" + Message + "</p>";

            msg.ReplyToList.Add(new MailAddress(EmailAddress));


            SmtpClient smtpClnt = new SmtpClient("smtp.ionos.co.uk");
            smtpClnt.UseDefaultCredentials = false;
            smtpClnt.Port = 587;
            smtpClnt.Credentials = new NetworkCredential(@"<email address>", "<password>");
            smtpClnt.EnableSsl = true;


            smtpClnt.Send(msg);


            lblError.ForeColor = System.Drawing.Color.Green;
            txtName.Text = "";
            txtEmail.Text = "";
            txtMsg.Text = "";
            txtPhone.Text = "";
            Response.Write("<script>alert('Message sent');</script>");
        }

        catch (Exception er)
        {
            string er1 = er.Message + "<br />" + er.InnerException;
            lblError.Text = er1;
            Response.Write("<script>alert('Something went wrong. Please try again later or call us on: 0<phone number>');</script>");
        }
    }
}

<script src='https://www.google.com/recaptcha/api.js'></script>  
<div class="g-recaptcha" data-type="image" data-sitekey="6LfDHwcpAAAAAL71bUOZ4291MqxqDyTfvGbzOMcg"></div>
<asp:Button ID="btnSend" runat="server" ValidationGroup="vldContact" Text="Send your message" CssClass="btn btn-danger" OnClick="btnSend_Click" />
My webconfig:

<system.net>
    <defaultProxy>
        <proxy usesystemdefault = "false" bypassonlocal="false" proxyaddress="http://ntproxyus.lxa.perfora.net:3128"/>
    </defaultProxy>
</system.net>

Failure sending mail.
System.IO.IOException: Unable to read data from the transport connection: An established connection was aborted by the software in your host machine. ---> System.Net.Sockets.SocketException: An established connection was aborted by the software in your host machine at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags) at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size) --- End of inner exception stack trace --- at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size) at System.Net.FixedSizeReader.ReadPacket(Byte[] buffer, Int32 offset, Int32 count) at System.Net.Security._SslStream.StartFrameHeader(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.Security._SslStream.StartReading(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.Security._SslStream.ProcessRead(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.TlsStream.Read(Byte[] buffer, Int32 offset, Int32 size) at System.Net.PooledStream.Read(Byte[] buffer, Int32 offset, Int32 size) at System.Net.Mail.SmtpPooledStream.Dispose(Boolean disposing) at System.IO.Stream.Close() at System.IO.Stream.Dispose() at System.Net.ConnectionPool.Destroy(PooledStream pooledStream) at System.Net.ConnectionPool.PutConnection(PooledStream pooledStream, Object owningObject, Int32 creationTimeout, Boolean canReuse) at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint) at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint) at System.Net.Mail.SmtpClient.GetConnection() at System.Net.Mail.SmtpClient.Send(MailMessage message)