If DNS is transiently down, the most common mail domains are still in local resolver cache. And if you're parsing live user requests, that means the IP network itself is not in transient failure at the moment. So it takes a pretty narrow kind of failure to trigger a problem... And the outcome is the app tells the user to recheck their email address, they do, and they retry and it works.

If DNS is having a worse problem, it's probably down for your mail server too, which means an email would at least sit in the outbound mail spool for a bit until DNS comes back. Meanwhile the user is wondering where their confirmation email is, because people expect email delivery in seconds these days.

So yeah ... yay, tradeoffs!

(Confirmation emails are still important for closed-loop opt-in, to make sure the user isn't signing someone else up for your marketing department's spam, though.)