This automation periodically re-checks the delivery status of your recently sent texts (both regular sends and marketing-campaign texts). If a message ended up failed or undelivered and hasn't already been refunded, the credits charged for it are automatically returned to your balance — you're not charged for texts that never actually reached anyone. If the real delivery cost differs from what was originally estimated (for example, it needed more or fewer segments than expected), it also corrects the charge to the right amount.