The Internet Message Access Protocol or IMAP is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being POP3. Virtually all modern e-mail clients and servers support both protocols as a means of transferring e-mail messages from a server, such as those used by Gmail, to a client, such as Mozilla Thunderbird, Apple Mail and Microsoft Outlook. Once configured, the client's use of such protocols remains transparent to the user.

E-mail protocols

Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol) is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on a remote server. The current version, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501.

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 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.