IMAP serves next generation of e-mail
POP, the Post Office Protocol, has been the
long-standing mail retrieval protocol for TCP/IP networks. But it's
started showing its age at a time when users are becoming more mobile,
accessing mail servers from a variety of locations, often using more
than one computer. And the shift from text-only e-mail to complex
messages containing graphics and multimedia attachments has further
strained the capabilities of POP.
New protocols are needed. One likely candidate is IMAP, or Internet
Message Access Protocol. Implementations of this protocol are finally
moving out of academic environments and other test beds to broader
commercial support, making it possible for corporate messaging systems
to shift to IMAP.
IMAP has been designed to pick up where POP left off, offering new
advantages. For instance, IMAP lets you routinely store your messages
on a central server and only request the messages you want to store
locally on their desktop or laptop computers. Additionally, IMAP
users can perform more manipulations on their messages at the
server, either by storing them in archive folders, requesting only
parts of a message, or sharing a mailbox with other users.
Let's take a closer look at how the two e-mail systems work.
Models of client/server e-mail
Both POP3 and IMAP4 (the latest versions of POP and IMAP, respectively) are protocols for handling
e-mail access. They each depend on another protocol, SMTP (Simple
Mail Transfer Protocol), for sending mail. To see some of the
differences between POP and IMAP, let's review the three basic
models of client/server e-mail--offline, online, and disconnected
use.
- The offline model is the most popular form of
client/server e-mail, and is used by protocols such as POP3. In this
model, a client application periodically connects to a server,
downloads all pending messages to the client machine and then deletes
these messages from the server. You process all of your mail locally on
your client computer.
- The online model is most commonly associated with
remote filesystem protocols, such as NFS. In this model, a client
application manipulates mailbox data on a server, maintaining a
connection throughout the session. The client stores no mailbox data
and only retrieves data from the server as needed. You cannot do
anything with your messages if you're disconnected from the server.
- The disconnected user model offers a hybrid of the
offline and online models. In this model, a user can download some set
of messages from the server, manipulate them offline, and then upload
the changes at some later time. The server is again the main repository
of the messages.
POP: A brief review
As mentioned, POP is the Internet's most popular e-mail
retrieval protocol today. It's a relatively simple protocol to
implement, and there are many client packages available for a variety
of computing platforms, including Unix, DOS, Windows, and Macintosh.
POP was designed to operate mainly in the offline mode and uses the
"store-and-forward" paradigm for handling your mail. You use your POP
client to connect to a mail server (the "store") and download all of
the pending mail to your own machine ("forwarding" the mail to your
client machine). Anything you do afterwards with your mail is
reflected only on your client machine, not on the server. And if you
use more
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