to ensure it is deliverable? Well, then you better send them an email.

I deal with user support for a site and I'd estimate at least 2% of our new users (>50 people PER DAY) enter wrong email addresses. Not "I forgot to put .com at the end" but "I thought my email was john.doe@gmail.com when it's actually john.doe@yahoo.com" which would pass validation with flying colours. The only real "solution" is to tell a user if the validation email has been sent yet (to deal with "well maybe I should wait 5 more minutes") and if it has and they don't have it allow them to change their email to their real email address. So many sites (incl. the one I manage) do not allow this, it's crazy.