Mailbox is used for inter-process communication for sending small messages between tasks or between ISR and task. The Mailbox is used in some Real Time Operating system (RTOS). If the task or thread want to send a message to another task or thread it creates the mailbox for sending the message. The message is sent by which task or thread and which created the mailbox is called mailbox server. And the message received by which task or thread is called mailbox client. Mailbox message is a kernel service.