Dunning is the recovery of failed recurring payments: retrying declined charges on an intelligent schedule and prompting customers to update their card details. In subscription businesses it is the machinery that decides how much involuntary churn, customers lost to payment failure rather than choice, actually sticks.
Why it matters
Payment failures quietly cancel more subscribers than dissatisfaction does, and most of those failures are recoverable. Each retry is a merchant initiated transaction against a stored card, so it must reference the original mandate or the acquirer may block it before the customer sees it. Good dunning reads the decline code before retrying: insufficient funds clears around paydays, expired cards need an update email not a retry, and hard declines must be left alone because schemes now fine excessive retries. Account updater services and network tokens fix expiring cards before they fail at all, which is cheaper than any recovery email.