SIRP based email System

In the SIRP based email delivery system, senders cannot directly push messages to arbitrary receivers. Instead, receivers decide if and when they want to retrieve (or pull) messages from senders. Figure 2 illustrates the basic architecture of the new email delivery system. In the following we will present the new system from both the senders' and receivers' perspectives. Before we delve into details, it is worth noting that the new system extends the current Simple Mail Transfer Protocol (SMTP) [9] by adding two new commands: MSID and GTML. In other words, all the commands and reply codes in SMTP are also supported in the new system. We will explain the two new commands when we use them.