It is common for modern ASIC to have multiple processors on chip. Mailbox is one way to support inter-processor communication. Mailbox can also be used for communication between processors on different chips. An interesting blog can be found at INTERPROCESSOR COMMUNICATION: Raspberry Pi 2 ARM-GPU IPC. It employs mailbox mechanism as below:
This drawing is simplified and the blog does mention how interrupt is used with mailbox.
Below is a generic mailbox structure.
Use points to gain access. You can either purchase points or contribute content and use contribution points to gain access.