I used to do that while at BrainLuxury. Also tried to put "I am not a robot " into the subject line, with mixed results. Around 5-10% of customers replied, which is probably not amazing, but not horrible either.
It seems perfectly plausible that 95% of the customers of an app or online don't want to enter into an email conversation about the service - they just want it to be usable without need for further communication, and they aren't interested in getting involved in your wider sales strategy either.