I think you're missing the point. The idea is to provide incremental knowledge about the validity of the address, both to the user and to the service. user@exis

I think you're missing the point. The idea is to provide incremental knowledge about the validity of the address, both to the user and to the service. user@existing.dns will validate but user@exsiting.dns won't. There is real value in providing that feedback immediately, rather than forcing the user to realize after waiting for an email that will never arrive that they made a trivial mistake. To be clear, I agree with you 100% that there is no substitute for sending a confirmation email and requiring the user to click-through if you're looking for full confirmation of mailbox validity. My point is that there are simple steps to be taken along that path which complement the confirmation email.