Foundations of Python Network Programming

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Chapter 15

IMAP

At first glance, the Internet Message Access Protocol (IMAP) resembles the POP protocol described in Chapter 14.
Plus, if you read the first sections of Chapter 13, which provide the entire picture of how e-mail travels across the
Internet, you will already know that the two protocols fill a quite similar role: POP and IMAP are two ways that a
laptop or desktop computer can connect to a remote Internet server to view and manipulate a user’s e-mail.
And that’s where the resemblance ends. Whereas the capabilities of POP are rather anemic—users can download
new messages to their personal computers—the IMAP protocol offers such a full array of capabilities that many users
sort and archive their e-mail permanently on the server, keeping it safe from a laptop or desktop hard drive crash.
Among the advantages that IMAP has over POP are:

•    Mail can be sorted into several folders, rather than having to arrive in a single in box.

•    Flags are supported for each message, like “read,” “replied,” “seen,” and “deleted.”

•    Messages can be searched for text strings right on the server, without having to download

each one.

•    A locally stored message can be uploaded directly to one of the remote folders.

•    Persistent unique message numbers are maintained, making robust synchronization possible

between a local message store and the messages kept on the server.

•    Folders can be shared with other users or marked read-only.

•    Some IMAP servers can present nonmail sources, like Usenet newsgroups, as though they

were e-mail folders.

•    An IMAP client can selectively download one part of a message, for example, grabbing a

particular attachment or only the message headers, without having to wait to download the

rest of the message.

Taken together, these features mean that IMAP can be used for many more operations than the simple
download-and-delete spasm that POP supports. Many e-mail readers, like Thunderbird and Outlook, can present
IMAP folders so that they operate with the same capabilities of locally stored folders. When a user clicks a message,
the e-mail reader downloads it from the IMAP server and displays it, instead of having to download all of the messages
in advance; the reader can also set the message’s “read” flag at the same time.