Perhaps you should interpret it as feedback that its not a hirer's market. In a market where most people have many opportunities the incentive to respond with a high effort response is very low. The better thing to do would be to consider the casual emails you get as leads to a potential hire - you'll have to put the effort in. Additionally what you are seeing is likely not by mistake - its an indication that hiring right now for applicants is in many cases more of a numbers game than it is about crafting a great cover letter. That being said I honestly don't think you are doing yourself much good with this sort of message - I'd just do the best that you can with what you get back.