The major difference between IMAP and POP is where mail is ultimately stored (and hence, how it is accessed). With POP, mail is transferred from a maildrop server to a local machine. IMAP, on the other hand, stores all messages on themaildrop.An IMAP-capable MUA passes messages back and forth to an IMAP maildrop to manipulate and view messages, but they are stored on the maildrop.

POP is therefore much more efficient in terms of network bandwidth. Since messages are transferred from a maildrop, there will seldom be a requirement to duplicate the message transfer. If a user wishes to see a stored message again, they may simply retrieve it from local storage. An IMAP user would be forced to retrieve the message over the network, at least once per session. Also for this reason, POP MUAs can be substantially faster than their IMAP counterparts.

Figure 10-2. POP usage for a dial-in LAN

Security and privacy aspects are often important when dealing with email messages. Many people are not comfortable with folders of private messages being stored on an Internet-connected server not under their direct control. After all, what you keep may reveal more about you than what you receive.

IMAP has its advantages, too. It was developed to support roaming users who connect from many different client machines. Storing mail on the server means that, once a user is properly authenticated, that user can get full access to all of her mail, both new and saved, regardless of the client that she happens to be using. It is a panacea for those that travel frequently and wish to maintain mail access without having to synchronize mailboxes later.