ESA Pipeline

The ESA pipeline has some stages that are fixed in order, apply to all messages, and other components that are either optional based on the message or completely optional for all messages. For example, spam filtering may not be performed on mail from certain senders, and virus scanning need not be performed at all on the pipeline. Direction of messages also dictates the order and stages that messages travel through. Messages that arrive at the ESA can pass through either an incoming pipeline or an outgoing pipeline, although we’ll see that this distinction is not always clear. Incoming and outgoing are not really separate pipelines or queues; in fact, they don’t refer to any distinct features, as all messages arriving at an ESA are treated more or less the same. They really only distinguish between which filtering steps occur and which don’t, and how the filtering affects the messages.