This article is from the LAN Mail Protocols FAQ, by John Wobus email@example.com with numerous contributions by others.
There are advantages to having a central server receive the mail
destined to desktop computers and having the desktop computer collect
the mail from the server on demand:
* Your desktop computer may be down quite a bit and less network
bandwidth and less of the processing resources of the sending
computer are used if the computer receiving your mail is ready to
* Some people use more than one desktop computer to read mail.
* A desktop computer may not have the resources to store all the
mail you receive.
* It can make your e-mail address more like other users'.
The easiest way to "implement" this is to run the central mail server
like any multi-user system: let people sign on to it and use some mail
utility. Then desktop computer users can use "terminal sessions" to
sign on to the central mail server and read their mail (e.g. with Unix
"pine"). This has the disadvantage of making the desktop computer
users learn and use the central mail server's procedures.
SMTP, the "internet" mail protocol used to deliver mail between
multi-user systems only supports mail transfer initiated by the sender
(actually, SMTP has a method to initiate reception, i.e. the "TURN"
command, but the method has problems, didn't catch on and is not
used). Other protocols have been devised to allow a desktop computer
to request transfer of mail, thus able to make use of a central
server. These include the published variants of POP, IMAP, and DMSP.