Email clients using IMAP generally leave messages on the server until the user explicitly deletes them.
Most email clients support IMAP in addition to Post Office Protocol (POP) to retrieve messages.
It went through a number of iterations before the current VERSION 4rev2 (IMAP4), as detailed below: The original Interim Mail Access Protocol was implemented as a Xerox Lisp Machine client and a TOPS-20 server.
[7][8] Although some of its commands and responses were similar to IMAP2, the interim protocol lacked command/response tagging and thus its syntax was incompatible with all other versions of IMAP.
When using POP, 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.
Multiple mailbox support also allows servers to provide access to shared and public folders.
This mechanism avoids requiring clients to download every message in the mailbox in order to perform these searches.
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.
The IMAP specification has been criticised for being insufficiently strict and allowing behaviours that effectively negate its usefulness.
[17] From an administrative and resource point of view, the IMAP protocol can be viewed as an early implementation of cloud computing, as the intent and purpose of IMAP is to maintain your mailbox structure (content, folder structure, individual message state, etc) on the mail server, whereas with POP, this is all maintained on the user's local device.
Thus, IMAP requires far more server side resources, incurring a significantly higher cost per mailbox.
Notification of mail arrival is done through in-band signaling, which contributes to the complexity of client-side IMAP protocol handling somewhat.
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.
In addition to this, Courier Mail Server offers a non-standard method of sending using IMAP by copying an outgoing message to a dedicated outbox folder.