Message credits are handled differently for paid accounts and the Forever Free plan.
Paid accounts - if you run out of message credits, new subscribers will still be able to join your Lists, and receive an auto-response messages. Your account will not be shut down, nor will you be billed for overage messages. However, when you login to your account, if your message balance is at or below 0, you will not be able to use your account until you resolve the issue by either upgrading your monthly plan or purchasing additional message credits. You're negative credit balance will be deducted from your next billing cycle.
Forever Free plan - when your message credit balance is below 0, new subscribers will not be able to join. You will need to upgrade your plan or purchase additional message credits to continue using your account.