On which type of computers does the back pressure system resource exist?

The back pressure system resource specifically exists on Hub Transport servers. This feature is crucial in managing message flow within an Exchange Server organization. When the Hub Transport server detects that its resources (such as CPU and memory) are being heavily utilized, it activates back pressure to prevent the server from becoming overloaded. By signaling downstream servers to reduce the volume of incoming messages, it ensures stable performance and prevents message loss or delays.

In contrast, file servers, application servers, and database servers serve different roles and do not utilize back pressure in the same way as Hub Transport servers. File servers primarily handle file storage and sharing, application servers run applications, and database servers manage data storage and retrieval. Therefore, they do not require the same mechanisms for managing mail flow and server resource utilization that are specific to Hub Transport servers in an Exchange environment. This makes the choice of Hub Transport servers the only correct answer for where the back pressure system resource exists.