/// <summary>
        /// Is called when email domain target servers resolve operation has completed.
        /// </summary>
        /// <param name="to">RCPT TO: address.</param>
        /// <param name="op">Asynchronous operation.</param>
        /// <exception cref="ArgumentNullException">Is raised when <b>to</b> or <b>op</b> is null reference.</exception>
        private void EmailHostsResolveCompleted(string to, Dns_Client.GetEmailHostsAsyncOP op)
        {
            if (to == null)
            {
                throw new ArgumentNullException("to");
            }
            if (op == null)
            {
                throw new ArgumentNullException("op");
            }

            if (op.Error != null)
            {
                LogText("Failed to resolve email domain for email address '" + to + "' with error: " + op.Error.Message + ".");

                Dispose(op.Error);
            }
            else
            {
                StringBuilder buf = new StringBuilder();
                foreach (HostEntry host in op.Hosts)
                {
                    foreach (IPAddress ip in host.Addresses)
                    {
                        m_pTargets.Add(new Relay_Target(host.HostName, new IPEndPoint(ip, 25)));
                    }
                    buf.Append(host.HostName + " ");
                }
                LogText("Resolved to following email hosts: (" + buf.ToString().TrimEnd() + ").");

                BeginConnect();
            }

            op.Dispose();
        }