Two possible approaches: First, run it through a parser to verify there's an @, a valid suffix (.com, .de, etc), resolvable domain, and just hope that they spe

Two possible approaches: First, run it through a parser to verify there's an @, a valid suffix (.com, .de, etc), resolvable domain, and just hope that they spelled it correctly. Second, send it an email (maybe just a "thank you for signing up"), and rather than waiting for the customer to respond, just listen for a "undeliverable as addressed" reply message, which should be pretty prompt, but not always. Probably not much else you can do, stopping short of a confirmation email request.