(Internet Message Access Protocol 4) A programming interface (API) from the IETF that enables a user's e-mail program to access the mail server (RFC 3501 standard). E-mail clients such as Outlook, Mail, Eudora and Thunderbird are typically configured to retrieve mail either via IMAP4 or POP3, the other popular standard. IMAP4 has more options than POP3. For example, it can download only the message headers, which have only to/from addresses and subject. An essential option in the early days of dial-up, the user then chooses which messages and attachments to download. IMAP also keeps the messages on the mail server in synchronization with the downloaded messages in the client. Users can then 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 IMAP4 or POP3. In addition, some users like to keep their e-mail online indefinitely for future searching. In such cases, IMAP4 offers the flexibility of selectively retaining messages, whereas POP3 does not. See POP3, e-mail interfaces, SMTP and messaging system.