IMAP supports both connected (online) and disconnected (offline) modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other facets of IMAP operation allow multiple clients to access the same mailbox. Most e-mail clients support either POP3 or IMAP to retrieve messages; however, fewer Internet Service Providers (ISPs) support IMAP. IMAP4 offers access to the mail store; the client may store local copies of the messages, but these are considered to be a temporary cache; the server's store is authoritative.
E-mail messages are usually sent to an e-mail server that stores received messages in the recipient's e-mail mailbox. The user retrieves messages with either a web browser or an e-mail client that uses one of a number of e-mail retrieval protocols. Some clients and servers preferentially use vendor-specific, proprietary protocols, but most support the Internet standard protocols, SMTP for sending e-mail and POP3 and IMAP4 for retrieving e-mail, allowing interoperability with other servers and clients. SMTP can also be used for retrieving email; it is more suitable for permanent Internet connection than, say, a dialup connection, and is not supported by most e-mail client software. For example, Microsoft's Outlook client uses a proprietary protocol to communicate with an Exchange server as does IBM's Notes client when communicating with a Domino server, but all of these products also support POP3, IMAP4, and outgoing SMTP. Support for the Internet standard protocols allows many e-mail clients such as Pegasus Mail or Mozilla Thunderbird (see comparison of e-mail clients) to access these servers, and allows the clients to be used with other servers (see list of mail servers).
E-mail clients can usually be configured to use either POP3 or IMAP4 to retrieve e-mail and in both cases use SMTP for sending. Most e-mail programs can also use Lightweight Directory Access Protocol (LDAP) for directory services.
Nearly all subscribers to individual Internet service provider e-mail accounts access their e-mail with client software that uses POP3.
IMAP is often used in large networks, for example, a college campus mail system. IMAP allows users to access new messages as fast as the network can deliver them to their computers. With POP3, users either download the e-mail to their computer or access it via the web. Both methods take longer than IMAP over a local network, and the user must download any new mail (e.g. by "refreshing" the page) to see the new messages.
No copies of the original interim protocol or its software exist; all known installations of the original protocol were updated to IMAP2. 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.
Some design flaws in the original IMAP4 (defined by RFC 1730) that came out in implementation experience led to its revision and replacement by IMAP4rev1 two years later. There were very few IMAP4 client or server implementations due to its short lifetime.
IMAP4rev1 is upwards compatible with IMAP2 and IMAP2bis; and is largely upwards-compatible with IMAP4. However, the older versions are either extinct or nearly so.
Unlike many older Internet protocols, IMAP4 natively supports encrypted login mechanisms. Plain-text transmission of passwords in IMAP4 is also possible. Because the encryption mechanism to be used must be agreed between the server and client, plain-text passwords are used in some combinations of clients and servers (typically Microsoft Windows clients and non-Windows servers). It is also possible to encrypt IMAP4 traffic using SSL, either by tunneling IMAP4 communications over SSL on port 993, or by issuing STARTTLS within an established IMAP4 session (see RFC 2595).
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.
The following e-mail services support IMAP access:
The following clients support IMAP on a Treo
Patent Application Titled "System and Method for Multiple Accounts to Access Internet Message Access Protocol Server" Published Online
Jun 20, 2013; By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington,...
WIPO PUBLISHES PATENT OF ZTE FOR "SYSTEM AND METHOD FOR MULTIPLE ACCOUNTS TO ACCESS INTERNET MESSAGE ACCESS PROTOCOL SERVER" (CHINESE INVENTOR)
Feb 18, 2012; GENEVA, Feb. 18 -- Publication No. WO/2012/019453 was published on Feb. 16. Title of the invention: "SYSTEM AND METHOD FOR...
Agency Reviews Patent Application Approval Request for "System and Method for Configuring Access to Electronic Mailboxes"
Oct 17, 2013; By a News Reporter-Staff News Editor at Politics & Government Week -- A patent application by the inventors GARDNER, DARREN LANCE...