Well, aside from webhooks or anything, Kickbox and its competitors seem to focus on checking if an email address exists based on DNS and SMTP pings.

This is valuable, but I want to be able to know for sure that 1. the email is "real" and 2. the person using the email address actually has access to it and can receive emails on it. Thus the process of actually sending an email to the provided address and making them click a link.

I've still not seen anything that provides this functionality.