TCP/IP Electronic Mail Mailbox Access Model, Method and Protocol Overview
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In an ideal world we would all be born knowing everything there is to know about computers and networking, and I'd have become a famous novelist instead of writing thousands of pages of this stuff. J Well, that may be asking a bit too much, but wouldn't it have been nice if every device on the Internet simply ran SMTP server software? If so, then that one protocol would be sufficient to implement the entire TCP/IP e-mail system. You would just compose e-mail on your machine, and your SMTP software would send it to your recipient's, and he or she would read it. Nice and simple.
Back here in the real world, however, this is really not possible in general terms. As I explained in the overview section on e-mail and the discussion of SMTP, an SMTP server must be connected to the Internet and available around the clock to receive e-mail sent at any time by any of the millions of other computers in the world. Most of us either cannot or do not want to run machines continuously connected to the Internet, nor do we want to configure and maintain potentially-complex SMTP software. This is the reason why a complete e-mail exchange normally involves not two devices but four: a message is composed on the sender's client machine, transferred to the sender's SMTP server, then to the recipient's SMTP server, and finally, to the recipient's machine.
The communication between SMTP servers is of course done with SMTP. So is the initial step of sending the e-mail from the sender's machine to the sender's SMTP server. However, SMTP is not used for the last part of the process, accessing the recipient's mailbox. Instead, a set of mailbox access and retrieval protocols and methods were devised.
A fair question is why was this done? Why not simply have mail sit pending on the recipient's SMTP server and then have it send the mail to the recipient client device when it comes online, using SMTP? There are two main reasons for this. The first is that SMTP was designed for the specific purpose of transporting e-mail only. Having it responsible for client mailbox access would require adding more functionality, making it difficult to keep SMTP simple. Also, SMTP works on a push model, with transactions being initiated by the sender. It would need changes to allow it to respond to requests from a client device that is only online intermittently.
But the second reason is probably more important: flexibility in how electronic mail is accessed. If we used SMTP, all we would be able to do is transfer e-mail to the recipient's client machine. This would be functional, but would greatly limit the capabilities of how e-mail is used. For example, some users might wish to access mail directly on the server and manipulate it there. Also consider the problem of people with special requirements, such as those who travel and may need to access e-mail from a number of different client devices.
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