Like in the current email architecture, a sender uses a Mail User Agent (MUA) to compose outgoing messages [9]. After a message is composed by the sender, the sender delivers the message to the sender Mail Transfer Agent (MTA). For simplicity, we refer to a sender MTA server as an SMTA, and a receiver MTA server as an RMTA.
All the outgoing messages are stored at the SMTA. For this purpose, the SMTA maintains an outgoing message folder for each sender. Instead of the complete message being directly pushed from the SMTA to the RMTA, only the envelopes (headers) of the messages are delivered. In particular, the SMTA notifies the RMTA about a new message by the new message identifier command MSID, which contains the unique identifier msid of the message. The identifier of a message is generated based on the sender, the message, the receiver, and a secret key of the sender.
We note that there is a fundamental difference between message pull in the new email delivery system and URL embedded in many current spam messages. The address in the URL is normally not related to the sending machine of the message, which makes it hard to identify the actual sender who is responsible for the spam message. On the other hand, outgoing messages in the new email system have to be stored on the sender mail servers instead of third-party machines before they are retrieved. In this way, we obtain several advantages in restricting spam. For example, senders need to keep their mail servers up until the messages are retrieved by receivers. This presents less flexibility for senders to move around by frequently changing their IP addresses and/or domains. In contrast, in the current (sender-push) SMTP-based architecture, spammers can send a large number of spam messages and shut down their mail servers, which makes it hard to hold spammers responsible for spamming. Moreover, in the new system, senders have greater responsibility to store and manage their outgoing email messages in comparison to the current email architecture, which imposes negligible responsibility on the senders. In summary, while the current SMTP-based email delivery architecture provides a call-by-copy interface to senders, the new system provides a call-by-reference interface to senders [5].