Microsoft Exchange Server 2013: Connectivity, Clients, and UM

Back pressure

The Exchange transport engine is designed to process extremely large volumes of messages, but sometimes a server might not have the physical capability to handle incoming or outgoing messages. For instance, a Mailbox server might come under load from another source that consumes much of the available memory, or the disk space on some drives might come close to being exhausted. In these circumstances, the back pressure feature enables the Transport service to continue running normally and process queued messages while it temporarily rejects incoming messages to stop new messages from being queued. In this scenario, the sending SMTP servers have to queue the messages until the situation that caused pressure on Exchange is relieved. ...