Eh, but consider online shopping carts. I go to some ecommerce site, enter in my email to buy something. If I have to verify the email first, conversion rates w

Eh, but consider online shopping carts. I go to some ecommerce site, enter in my email to buy something. If I have to verify the email first, conversion rates will plummet. Instead, (and this is what we do at my company), we attempt to validate that the email is a real one with MX lookups and even SMTP checks. But otherwise, you just have to trust the shopper.