You have a list of addresses. Maybe they came from a signup form, a partner CRM, an enrichment run, or last quarter's webinar. You want to know which ones are real before you put them on a campaign and torch your sender reputation. You search "email verification" and find a hundred services with landing pages claiming 99% accuracy. You install the obvious package, run it on your list, and 95% come back "valid." You send. A quarter of them bounce, and the major inbox providers start flagging your domain. What happened? Some combination of: syntax-valid addresses that don't exist, mail servers that lie, catch-all domains, greylisting, and anti-probe behavior. "This is a real mailbox" is much harder to prove than it looks. Here's how the protocol actually works, where it breaks, and what a serious verifier has to do about it. Syntax checks filter the obvious garbage and nothing else Run a regex against [email protected] and you'll catch the obvious malformed strings.…