A brief analysis of IMAP protocol

IMAP (Internet Message Access Protocol) is another protocol that corresponds to the POP3, a multiple mailbox e-mail system developed by Stanford University in the United States in 1986. It has the advantages of high performance and scalability by getting information about e-mail from the mail server or by directly collecting mail. IMAP for many client-side e-mail software, such as OutlookExpress, Netscapemessenger, and so on, support IMAP server side of the software also more and more, such as Criticalpath, Eudora, IPlanet, SendMail and so on.

Readers can not help but ask, POP3 is also the agreement to receive mail, now is not used very well, why also use IMAP agreement?

Deficiencies in the POP3 protocol

Indeed, pop is the first offline protocol standard for messages on the Internet, allows users to download messages from the server to the local host, while deleting messages that are saved on the mail server, thereby eliminating the need for users to connect to the mail server for a long time, greatly reducing the overall cost of the server and the network.

But POP3 has its inherent flaw that when a user receives an e-mail message, all the letters are purged from the server and downloaded to the client. In the whole process of receiving, the user can not know the specific information of the message, only after the full income of the hard drive, can slowly browse and delete. This gives the user little control over the acceptance of the message. Once the mailbox is bombed, or have a relatively large mail, users can not analyze the content of the message and sender address to decide whether to download or delete, resulting in the waste of system resources. The IMAP protocol can not only overcome the defects of POP3, but also provide more powerful functions.

The analysis of IMAP

Three modes of IMAP-provided operations

Online: Messages remain on the mail server and can be managed by clients. It is used in the same way as webmail.

Offline: Messages remain on the mail server and can be managed by clients. This is the same as the POP protocol.

Detach mode: Part of the message is on the mail server side and part on the client. This is similar to the way some mature component packages are applied (such as Lotusnotes/domino).

The working principle and characteristic of IMAP

In an online manner, IMAP allows users to access and manipulate information on mail servers just as they would access and manipulate local information. IMAP software supports the arbitrary drag of mail between local folders and server folders to store files on the local hard disk on the server or retrieve the files on the server locally, all with a single mouse drag and drop operation.

On the client side, you can set up any hierarchy of folders on the server's mailboxes, and you can flexibly move messages between folders, mark the messages that you read or reply to, and delete files that are useless to you.

The summary browsing feature provided by IMAP allows you to read all the message arrival time, subject, sender, size and other information, while also enjoying the optional download attachment service. For example, an e-mail message contains 3 attachments, and only 1 of them are required, you can choose to download only these 1 attachments. You can fully understand before making whether to download, is all download or only part of the decision to download, so that users will not download spam because of the use of valuable space and waste of network fees.

IMAP also provides server-based mail processing and shared mail mailbox features. messages, including copies of downloaded messages, remain in the server until you delete them manually, which facilitates the generation and sharing of mail files. Users can view messages on the server on any client. This makes it easy for those roaming users to feel comfortable.

And IMAP, like POP3, allows users to download information from servers to their computers, which means they can still read messages offline.

In a detached state, the message state on the local system and the state of the message on the server may not be the same as when you reconnect later. At this point, the IMAP synchronization mechanism solves this problem. IMAP mail client software can record the user's local operation, when they connect to the network will send these operations to the server, the server will also tell the client software, when the user offline, when the server-side events, such as the arrival of new mail, to maintain the server and client synchronization.

In IMAP, you can define shared folders for use by other users with special access rights, while using POPs does not allow you to share mail boxes and share messages, only by copying or sending messages by hand. Sharing mailboxes makes it easier to work with an Internet mail-focused workgroup.

IMAP also offers a number of special features such as building subdirectories and accessing Usenet via IMAP. In the system administrator context, IMAP also provides a full set of features that are available.

The IMAP listening port is 143, the internal time and date of the message is given by the server, not the time and date given by the RFC822 Citic header, the true date and time when the message finally arrives. If the information is posted by an IMAP copy command, this should be the intrinsic time and date of the source information, and if the information is posted by the Iamp append command, this should be the time and date specifically described by the append command.

Many commands are defined in the IMAP protocol and can be performed using Telnet, such as Authenticate, list, and close, which are no longer detailed here.

Realize the lack of IMAP

IMAP is inferior to POP3 in leveraging server disk resources. Because the server-side messages are downloaded to the client while the pop is being used, they are deleted, so no extra space is used to store the old messages. IMAP servers keep old messages, occupy extra space, and need to delete old messages on a regular basis.

At the same time, because the user looks up the information title and decides which attachments to download, it also takes a certain period, so the link time is also longer than the pop way.

In application, because of the complexity of IMAP, developers to develop server and client software brings some problems. For ISPs, the adoption of IMAP means that they have to spend money to buy the relevant business software, while paying high technical support costs, so the commercial implementation of the program is not much.