Two other quick points to add to the above suggestions. Questions regarding what made them buy the product or service in the first place will give you a lot of insight as well. For example: What made you choose our product over others available? You can send the question open ended to a small percentage of your 10,000 customers and provide a multiple choice option for the remainder (or once you get some insight from the open ended responses). Secondly, understanding what customers were doing as a workaround before purchasing your product will give insight into how to best target future customers, so include questions regarding this as well.