Filtering C# lambda

Hello Experts,
Following code passes EmployeeNumber one by one and if that is found in the  internalEmployeelist
then that does not become part of results.

results=results.Where(itm => !IsInternalemployee(itm.EmployeeNumber));  //results is iqueryable
private bool IsInternalemployee(String itm)
{
  return internalEmployeelist.Contains(itm) ? true : false; //internalEmployeelist is list<String>
}

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Please help in  providing some better way of doing it. please note that results is iqueryable while internalEmployeelist is just list<String>