[wtmt]

That's downright silly and a user hostile move, IMO (why at all wouldn't you want someone to test things out without having to give one of their main email address?). Your solution seems to assume that everybody uses only Gmail and Gmail plus addressing. Gmail also considers dots/periods in addresses as not existing. Try blocking those too (no, actually don't try this!). There are so many temporary or disposable email services that you'd be wasting your time trying to disallow all those. Your time could instead be better spent on making your product or service more attractive to paying customers.