I suppose it depends on what we mean by validate. Running an ecommerce site, I got a lot of mileage out of prompting the customer to fix emails that "looked wrong". We allowed them to proceed if they wanted. A really common one was "user@gnail.com" when "user@gmail.com" was wanted. We used a slightly modified version of https://github.com/mailcheck/mailcheck and found it to be really useful.
