Azure Service Bus can be leveraged to achieve fault tolerance and high availability in a messaging system by utilizing features such as duplicate detection and message ordering. Duplicate detection ensures that messages are only processed once, preventing duplications and maintaining data integrity. Message ordering enables the processing of messages in the order they were received, facilitating guaranteed sequential processing. Additionally, Service Bus provides auto-forwarding, allowing messages to be automatically redirected to an alternate location in case of system failures. By utilizing these capabilities along with appropriate scalability options, such as partitioned queues or topics, developers can design a robust messaging system that can handle failures and ensure uninterrupted operations.

Another approach to leveraging Azure Service Bus for fault-tolerant messaging is by utilizing the geo-disaster recovery feature. This feature allows for the replication of Service Bus entities across different Azure regions, ensuring that even in the event of a regional outage, the messaging system remains available. By setting up geo-disaster recovery asynchronously, messages sent to one region can be automatically replicated to a backup region, with the ability to failover if needed. This ensures that critical messages are not lost or disrupted even in the face of catastrophic failures. Additionally, by utilizing Azure Traffic Manager, the application can be configured to automatically failover to the backup region in case of an outage, further enhancing the system's resiliency. Through these measures, developers can leverage Azure Service Bus to design a fault-tolerant messaging system that can withstand unforeseen challenges and maintain continuous operations.

In addition to the features mentioned, Azure Service Bus also offers transactional support, which enables atomic message processing within a transactional scope. This ensures that messages are either successfully processed by all the receivers or rolled back if any error occurs during the processing. Furthermore, Service Bus provides a dead-letter queue mechanism, where messages that cannot be processed or require manual intervention can be moved to a designated queue for investigation and debugging. This helps in identifying and rectifying issues that might impact the normal functioning of the messaging system. By utilizing these advanced capabilities and combining them with Azure's built-in monitoring and alerting features, developers can create a fault-tolerant and highly available messaging system that can deliver reliable communication across distributed applications and services.