In short: "It can't be done"

To elaborate; The only way you can tell reliably if an email was recieved, is to see if it is responded to. This also covers the case of people entering other users email addresses, too.

To do all of this in the timescale of a HTTP request just isn't going to happen.

My suggestion, as I've implemented before, is do everything in your best interests to see if it's worth trying to send an email (is it syntactically correct, is there an MX record, etc) and email a validation link with a secret in it. -- This seems to be the sort of thing you were suggesting.

In my experience, end users do twig when the email they've asked for hasn't arrived and sign up again with the email address corrected -- maybe you could look at writing an auto-bounce handler later to flag a warning if they try with that email address again?

--
RatArsed

In reply to Yet another comment on Yet Another E-mail Validation Question by RatArsed
in thread Yet Another E-mail Validation Question by tanger

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