For paid accounts, when the number of active subscribers exceeds the current subscriber limit, the subscriber limit will automatically increase and your credit card will be charged for the next pricing tier. You will receive a notification in your account when the limit is close to being exceeded. For free accounts, when the number of active subscribers exceeds the 1,000 active subscriber limit, sendings will be locked (campaigns + automation). Clients will be prompted to sign up for a paid account, or they will need to remove active subscribers to regain sending ability.