Sharepoint: Lookup against a large list with CAML Query

I have a SharePoint List (list_dls) which has all the Email addresses. The list has a single column with Field name “Title”. I need to validate that the Item in the “LookupField” is contained in the ‘list_dls’. The list has over 5000 emails, hence I will need to do a CAML query. The script does not seem to find the item. Any ideas to get it to work would be really appreciated.

Here is the code.

<input type='button' value='get Lists' onclick="PopulateLookupField();"/>
<p id="demo"></p>
<script language="javascript" src=""></script>
<script language="javascript" type="text/javascript">

var LookupField = '';

function PopulateLookupField() 
    var clientContext = new SP.ClientContext.get_current();
    var LookupList = clientContext.get_web().get_lists().getByTitle('list_dls');
    var camlQuery_list = new SP.CamlQuery();
    camlQuery_list.ViewXml = @"<View Scope='RecursiveAll'><ViewFields><FieldRef Name='ID'/><FieldRef Name='Title'/></ViewFields><RowLimit Paged='TRUE'>5000</RowLimit></View>";
    LookupList<ListItem> allListItems = new LookupList<ListItem>();
        var listItemCollection = LookupList.GetItems(camlQuery_list);
        var ItemFound = listItemCollection.Where(item => item.FieldValues["Title"].ToString().Contains(LookupField); 
        if(ItemFound !== null){break;}
        camlQuery_list.ListItemCollectionPosition = listItemCollection.ListItemCollectionPosition;
    } while (camlQuery_list.ListItemCollectionPosition != null);
    alert("ItemFound = " + ItemFound);