Once a bad address makes it onto your list, cleaning up the damage later is expensive. Far better to stop it at the door.
The flow
Before your form submits, send the address to the verify endpoint; if it doesn't come back deliverable, warn the user on the spot.
POST /api/v1/verifyThe response includes result, score, and the details of the MX, SMTP, and spam-trap checks. Credits are drawn from the same balance as the dashboard, and you can get a webhook notification the moment a job finishes.