The new email delivery system grants more control to receivers regarding if and when receivers want to read a message, senders cannot arbitrarily push a message to them. Receivers can be discriminate about which messages need to be retrieved, and which ones need not. If the receiver indeed wants to read a message, he will inform his own RMTA, and the RMTA will retrieve the message from the SMTA on behalf of the receiver. An RMTA retrieves an email message using a get mail command GTML, which includes the identifier msid of the message to be retrieved. After the message has been pulled to the RMTA, conventional virus/worm scanning tools and content-based spam filters can be applied to further alert the receiver about potential virus or spam. Therefore, the new email delivery system does not exclude the use of existing email protection schemes. For security reasons, when an SMTA receives the GTML command, it needs to verify that the corresponding message is for the intended receiver, and more importantly, the requesting MTA is the mail server responsible for the receiver (i.e. the one which was originally contacted for message delivery).
By only delivering the envelope (including msid) of a message from a sender to the receiver, less bandwidth, storage, and processing time is used at the receiver side, which is especially important for resource constrained users, e.g., wireless, PDA, or dial-up users. On the other hand, if the receiver indeed wants to read the message, negligible extra time and bandwidth is required. Since the receiver is less likely interested in messages from unknown sources, the majority of such messages will not be retrieved. As a result, considering the huge volume of spam on the Internet, much less bandwidth will be wasted by spam. For simple back of envelope calculation, assuming there are 30 billion spam messages sent daily on the Internet [10] and the average size of these messages is 5 KBytes [6]. We further assume the envelope of these messages occupies 1KBytes on average. Then it is easy to see that we will have daily 120 Tera Bytes worth of bandwidth saving on the Internet. Note that if content-based filter is used alone, these spam messages are still delivered on the Internet.