In the response, thank the user for initiating registration and send her an email with a link (and a token) to continue with the process. Of course, the downsi

In the response, thank the user for initiating registration and send her an email with a link (and a token) to continue with the process. Of course, the downside is that you're slowing the user down. It's acceptable for the sites that choose to require valid email addresses: if you're going to go there you might as well get it done sooner. You'd also be spamming potential victims, but that may not be that bad, as you'd also be alerting them.