Few days ago, I was searching on Google for exact this tool, problem was apollo flagged some issues and I needed to find out all of those quickly. So your app has a real use case. Just one question, normally people will not want to have a subscription for it, this is a validation they need once or twice, so maybe you can consifer a different pricing plan as well.
Thanks for the feedback! The domain checker tool is completely free and unrelated to the subscription. The subscription is for dmarc monitoring (I collect the dmarc aggregation reports and provide insights). I'll see if I can make that more clear.