There are several administrative tasks associated with using message stores for some or all email recipients:
Selecting and designating a mail server for the message store. For large sites, this task expands to designing and deploying a scheme in which several servers divide this task.
Configuring daemons to run POP and/or IMAP on the mail servers.
Setting up user mail programs to access the message store instead of or in addition to the default local mailbox.
The first item is intimately related to the overall network architecture and capacity planning, and this issue is discussed in this context in Chapter 15.
The second item deals with providing server-side support for remote email clients wishing to access and retrieve messages. There are two main protocols used for this purpose: the Post Office Protocol Version 3 (POP3, or justPOP) and theInternet Message Access Protocol Version 4 (IMAP4, or just IMAP).
POP is the older of the protocols, and it is also simpler than IMAP . It was designed for “offline” mail processing; the user’s mail program connects to the mail server and downloads any new mail messages to the local system (usually deleting them from the server after doing so). In this scheme, the remote server functions purely as a temporary remote storage site. Although POP clients can be configured to automatically poll the mail server periodically, POP remains a manual transfer method at heart.
IMAP implements an interactive client-server model ...