In some usecases, it would increase conversion if the user is not asked to enter the email. For example, if you are selling a privacy product and do not collect want to emails in general. Another example is when Paypal is the only checkout option, so instead of asking the user for the email, just send it to the Paypal email.

If no email is provided by the gateway, just don't send it, the user will still be able to receive his goods by being redirected to the "product received" page.