Message credits are handled differently for paid accounts and the free plan.
Paid plan - if you run out of message credits, new Subscribers will still be able to join your Lists, and receive auto-response messages. Your account will not be shut down, you won't be upgraded automatically, 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 be required to resolve the issue by either upgrading your monthly plan or purchasing additional message credits.
Free plan - when your message credit balance is at 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.