My campaign didn’t mail due to insufficient funds, how can I fix this?

First head to your campaign, you'll see an Exceptions tab has appeared next to the History tab. Under the Exceptions tab you'll see the the number of stalled mailers, and the balance required to push them through.

Go to the Billing page and manually add enough credit to your account to cover the rest of the mailers. Once your account is sufficiently funded, head back to the Exceptions tab and you'll see "Insufficient Funds" has turned into a Retry button. Click the button once and you'll notice the number of Exceptions start to go down as the mailers process.