I'm pretty new to email marketing, but this product has caught invalid emails and I was able to check through the list and spot that perhaps a comma was used instead of a dot, or maybe an extra letter was smashed in, and then I adjust and resubmit the email through the verifier and found I've saved a bunch of subscribers that would have not gotten their newsletter through this product! There's a peace of mind to knowing the emails I'm adding to my list aren't spam bots or something. My only warning is not to throw out the ones that are lumped into the "catch all" category. I've found that many of my active subscribers have older accounts that got grandfathered into the newer services, and they get to keep their old email addresses. They are still safe and good, but get lumped into "catch all" category because they don't follow the normal checks with an old email. Great product, definitely recommend!