Please don't continue to run this. What you are doing is effectively recreating the VRFY command that nearly every major mail hosting service has removed for privacy and abuse reasons. You are opening yourself up to a huge liability, since spammers will quickly use stolen credit cards to crosscheck their lists against your API. This will cause you to have processing issues from Stripe. This will cause you to have a h…
The person doing this doesn't care about the ethics of this. He's got his passive income - he's going to take it and run.
Since this is going to get shut down for abuse, it's not exactly passive.