The system attempts to send an invoice to customers four times before their account gets deactivated. After the fourth failed attempt, the customer's account gets locked until due bills are fully paid. To gain access once more, customers must resubscribe by logging into their accounts. 

As an operator, you can allow customers to remain active even if they haven't paid their overdue bills by clicking on the button titled "Activate [user]" next to their account. If the account remains active without a valid credit card or sufficient funds, SphereMail is not responsible for future bill collections.