Most signup forms validate an email with a regex and call it done. That catches bob@@gmail but happily accepts [email protected] , [email protected] and [email protected] . A month later your welcome emails are bouncing and your sender reputation is paying for it.

Here's a small, practical pattern for checking addresses at signup without blocking real users. The code uses Node.js and the ListCheckup API (disclosure: I work on it), but the decision logic at the end applies to any verifier.

What a real check tells you

A proper verifier goes beyond syntax: it looks up the domain's MX records and asks the recipient's mail server whether the mailbox exists, without sending anything.…