Internet Message Access Protocol, or its acronym IMAP according to AbbreviationFinder.org, is a network protocol for accessing electronic messages stored on a server. Using IMAP, you can access email from any computer that has an Internet connection. IMAP has several advantages over POP, which is the other protocol used to get mail from a server. For example, it is possible to specify server-side folders in IMAP. On the other hand, it is more complex than POP since it allows you to view messages remotely and not downloading messages as POP does.
IMAP and POP3 (Post Office Protocol version 3) are the two most prevalent protocols for obtaining email. All email servers and clients are virtually supported by both, although in some cases there are some typically proprietary vendor-specific interfaces. For example, the proprietary protocols used between the Microsoft Outlook client and your Microsoft Exchange Server or the IBM Lotus Notes client and the Domino server. However, these products also support IMAP and POP3 interoperability with other clients and servers. The current version of IMAP, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501.
IMAP was designed as a modern alternative to POP by Mark Crispin in 1986. Fundamentally, the two protocols allow mail clients to access messages stored on a mail server.
Whether using POP3 or IMAP4 to get messages, clients use SMTP to send messages. Email clients are commonly called POP or IMAP clients, but SMTP is used in both cases.
Most mail clients use LDAP for their directory services.
IMAP is frequently used in large networks; for example the mail systems of a campus. IMAP allows users to access new messages instantly on their computers, since the mail is stored on the network. With POP3, users would have to download the email to their computers or access it via the web. Both methods take longer than IMAP would, and you have to download the new email or refresh the page to see the new messages.
Unlike other Internet protocols, IMAP4 allows native encryption mechanisms. Plain text password transmission is also available.
Advantages over POP3
Some of the important features that differentiate IMAP and POP3 are:
Support for online and offline modes of operation
When using POP3, clients connect to the mail server briefly, only as long as it takes them to download new messages. By using IMAP, clients stay connected as long as their interface is up and download messages on demand. This way of working for IMAP can give faster response times for users who have a large number of messages or large messages.
Support for the connection of multiple simultaneous clients to the same recipient
The POP3 protocol assumes that the connected client is the sole owner of an email account. In contrast, the IMAP4 protocol allows simultaneous access to multiple clients and provides certain mechanisms for clients to detect changes made to a mailbox by another concurrently connected client.
Support for access to MIME parts of messages and partial fetching
Almost all email on the Internet is transmitted in MIME format. The IMAP4 protocol allows clients to obtain any individual MIME part separately, as well as to obtain portions of the individual parts or the entire messages. It is more secure.
Backup for message status information to be kept on the server
Through the use of signals defined in the clients’ IMAP4 protocol, the status of the message can be monitored, for example, whether or not the message has been read, answered or deleted. These signals are stored on the server, so that several clients connected to the same mail at different times can detect the changes made by other clients.
Backup for multiple access to mailboxes on the server
IMAP4 clients can create, rename, or delete mail (usually presented as folders to the user) on the server, and move messages between mail accounts. Support for multiple mailboxes also allows the server to provide access to shared and public directories.
Support for server-side searches
IMAP4 provides a mechanism for clients to ask the server to search for messages according to a variety of criteria. This mechanism prevents clients from downloading all messages from their mailbox, thus speeding up searches.
Support for a defined extension mechanism
Reflecting experience in previous versions of Internet protocols, IMAP defines an explicit mechanism by which it can be extended. Many IMAP4 extensions have been proposed and are in common use.
An example of an extension is the IMAP IDLE, which is used for the server to notify the client when a new email message has arrived and they are synchronized. Without this extension, to perform the same task, the client would have to periodically contact the server to see if there are new messages.