Oct 25, 2007


There are two main email protocols POP and IMAP. They have both been around for a long time and each have their advantages. But nowadays I am really feeling IMAP and I like that I can access my email via Web-mail, Outlook or my Windows Mobile Device and See THE SAME THING!

Lets start out by saying the Pop3 protocol is very 1999, but it's still very popular today. IMAP has
just so many wonderful features, I think more users would switch if they knew about them..

IMAP allows for two way communication between client and server. Think of it like a thin client. What's a thin client? Everything you do happens on the server. So if you have multiple clients in different locations, all your folders (including deleted and sent) will stay up to date and synced.

POP3 messages always need to be downloaded before being displayed. IMAP simply shows you the messages as they are on your mail server.

Gmail has given some users the IMAP option already. And IMAP works on most clients including the coveted iphone. Play on playa!

Advantages over POP3

Connected and disconnected modes of operation

When using POP3, clients typically connect to the e-mail server briefly, only as long as it takes to download new messages. When using IMAP4, clients often stay connected as long as the user interface is active and download message content on demand. For users with many or large messages, this IMAP4 usage pattern can result in faster response times.

Multiple clients simultaneously connected to the same mailbox

The POP3 protocol requires the currently connected client to be the only client connected to the mailbox. In contrast, the IMAP protocol specifically allows simultaneous access by multiple clients and provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients.

Access to MIME message parts and partial fetch

Nearly all internet e-mail is transmitted in MIME format, allowing messages to have a tree structure where the leaf nodes are any of a variety of single part content types and the non-leaf nodes are any of a variety of multipart types. The IMAP4 protocol allows clients to separately retrieve any of the individual MIME parts and also to retrieve portions of either individual parts or the entire message. These mechanisms allow clients to retrieve the text portion of a message without retrieving attached files or to stream content as it is being fetched.

Message state information

Through the use of flags defined in the IMAP4 protocol clients can keep track of message state, for example whether or not the message has been read, replied to, or deleted. These flags are stored on the server, so different clients accessing the same mailbox at different times can detect state changes made by other clients. POP3 provides no mechanism for clients to store such state information on the server so if a single user accesses a mailbox with two different POP3 clients state information, such as whether a message has been accessed, cannot be synchronized between the clients. The IMAP4 protocol supports both pre-defined system flags and client defined keywords, System flags indicate state information such as whether a message has been read. Keywords, which are not supported by all IMAP servers, allow messages to be given one or more tags whose meaning is up to the client. Adding user created tags to messages is an operation supported by some Webmail services, such as Gmail, although generally not using IMAP.

Multiple mailboxes on the server

IMAP4 clients can create, rename, and/or delete mailboxes (usually presented to the user as folders) on the server, and move messages between mailboxes. Multiple mailbox support also allows servers to provide access to shared and public folders.

Server-side searches

IMAP4 provides a mechanism for a client to ask the server to search for messages meeting a variety of criteria. This mechanism avoids requiring clients to download every message in the mailbox in order to perform these searches.

Built-in extension mechanism

Reflecting the experience of earlier Internet protocols, IMAP4 defines an explicit mechanism by which it may be extended. Many extensions to the base protocol have been proposed and are in common use. IMAP2bis did not have an extension mechanism, and POP3 now has one defined by RFC 2449.

Disadvantages of IMAP

While IMAP remedies many of the shortcomings of POP, this inherently introduces additional complexity. Much of this complexity (e.g., multiple clients accessing the same mailbox at the same time) is compensated for by server-side workarounds such as maildir or database backends.

Unless the mail store and searching algorithms on the server are carefully implemented, a client can potentially consume large amounts of server resources when searching massive mailboxes.

IMAP4 clients need to explicitly request new email message content potentially causing additional delays on slow connections such as those commonly used by mobile devices. A private proposal, push IMAP, would extend IMAP to implement push e-mail by sending the entire message instead of just a notification. However, push IMAP has not been generally accepted and current IETF work has addressed the problem in other ways (see the Lemonade Profile for more information).

Unlike some proprietary protocols which combine sending and retrieval operations, sending a message and saving a copy in a server-side folder with a base-level IMAP client requires transmitting the message content twice, once to SMTP for delivery and a second time to IMAP to store in a sent mail folder. This is remedied by a set of extensions defined by the IETF LEMONADE Working Group for mobile devices: URLAUTH (RFC 4467) and CATENATE (RFC 4469) in IMAP and BURL (RFC 4468) in SMTP-SUBMISSION. POP3 servers don't support server-side folders so clients have no choice but to store sent items on the client. Many IMAP clients can be configured to store sent mail in a client-side folder. In addition to the LEMONADE "trio", Courier Mail Server offers a non-standard method of sending using IMAP by copying an outgoing message to a dedicated outbox folder.[WikiPedia]

Originally Posted on Ask The Admin By Karl Gechlik

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