private static List<string> GetSubdomains(string websiteDomain, int startPageIndex = 1, int pageCount = 999, int pageSize = 14)
        {
            var list = new List<string>();
            //using bind to search subdomains in a certain website
            var bingSearchUrlFormat = "http://www.bing.com/search?q=site%3a{0}&go=Submit&qs=n&pq=site%3a{0}&sc=1-100&sp=-1&sk=&cvid=a9b36439006f4b05b09f9202c5b784bd&first={1}&FORM=PQRE";

            WebClient client = new WebClient();
            client.Encoding = Encoding.UTF8;
            var doc = new HtmlDocument();

            var first = (startPageIndex / 10) * 140 + 1;
            var stopIndex = first + pageCount*pageSize;
            var currentPageIndex = startPageIndex;
            for (var startItemSquenceNumber = first; startItemSquenceNumber < stopIndex; startItemSquenceNumber = startItemSquenceNumber + pageSize)
            {
                var response = client.DownloadString(string.Format(bingSearchUrlFormat, websiteDomain, startItemSquenceNumber));
                HtmlDocumentExtensions.LoadHtml2(doc, response);
                var docNode = doc.DocumentNode;
                var subDomains = docNode.QuerySelectorAll(".sb_meta cite");foreach (var subDomain in subDomains)
                {
                    list.Add(subDomain.InnerText);
                }
            }return list;
        }

        private static List<HtmlNode> GetWebPageNodes(string url, string elementSelector, string attributeNameContained, string attributeNameContainedValueLike)
        {
            var client = new WebClient();
            client.Encoding = Encoding.UTF8;
            var response = client.DownloadString(url);
            var doc = new HtmlDocument();
            HtmlDocumentExtensions.LoadHtml2(doc, response);
            var docNode = doc.DocumentNode;
            var emailNode = docNode.QuerySelectorAll(elementSelector).Where(node => node.Attributes.Where(attr => attr.Name == attributeNameContained).FirstOrDefault().Value.Contains(attributeNameContainedValueLike)).FirstOrDefault();

            var nodes = (from node in docNode.QuerySelectorAll(elementSelector)
                         where node.HasAttributes && node.GetAttributeValue(attributeNameContained, string.Empty).Contains(attributeNameContainedValueLike)
                         select node).ToList();

            return nodes;
        }

var subdomains = GetSubdomains("b2b.sample.com", stopPageIndex, 10);
var urlFormat = "http://{0}/contactus.html";
GetWebPageNodes(string.Format(urlFormat, item), "body table a", "href", "mailto").FirstOrDefault();