Check email addresses

Checks one address or up to 50 the way sending does before the first email to someone: the format, this project's suppression list (an earlier hard bounce or spam complaint, or an unsubscribe), likely typos of big mail domains (gmial.com), throwaway domains, and whether the domain exists and accepts email (MX, or an A record without MX). Sends nothing and stores nothing. Use it on addresses from a form or an import before sending. At most 30 calls per minute per project.