MaxHeaderLines is an Integer property that indicates the maximum number of header lines allowed when retrieving headers in an HTTP response. MaxHeaderLines is used to ignore server responses containing an inordinately large number of header lines (commonly referred to as "header bombing"). 

The default value for MaxHeaderLines is Id_TIdHTTP_MaxHeaderLines. The value in MaxHeaderLines can altered if you are experiencing header truncation with chatty servers. 

MaxHeaderLines is used as an argument when retrieving headers with the TIdHTTPProtocol instance used during HTTP protocol exchanges.

Id_TIdHTTP_MaxHeaderLines TIdHTTPProtocol.RetrieveHeaders