(Internet Message Access Protocol) A standard interface between a user's e-mail program and the mail server, as defined by IETF RFC 3501. IMAP4 and POP3 are the two common mailbox access protocols for e-mail clients such as Outlook, Mail, Eudora and Thunderbird.|
IMAP can be configured to download only the headers, which display to/from addresses and subject. An essential option in the days of dial-up, the user then chooses which messages and attachments to download. In addition, IMAP keeps the messages on the mail server in synchronization with the downloaded messages in the client. The user can decide when and which messages to delete on the server.
IMAP Flexibility for Web Mail
Although mail services provide a Web browser interface for managing mail, users often prefer to get mail via their favorite e-mail client, configured for IMAP or POP. In addition, some users like to keep their e-mail online indefinitely for future searching. In such cases, IMAP offers the flexibility of selectively retaining messages, whereas POP does not. See POP3, Internet e-mail service, SMTP and messaging system.
Most Internet mail services offer all three options. IMAP provides the most flexibility. Users can store their messages locally and have it synchronized with the server. In addition, IMAP lets multiple users access the same mailbox simultaneously.