IMAP

IMAP, or Internet Message Access Protocol, is an Internet protocol used by email clients to retrieve and manage messages stored on a mail server. Unlike POP, which generally downloads and may remove messages from the server, IMAP keeps messages on the server and allows multiple clients to access the same mailbox while maintaining a synchronized state. It supports server-side organization into folders (mailboxes), server-side searching and filtering, and the ability to manipulate message flags and status without downloading the full content.

IMAP4rev1, defined by the Internet Engineering Task Force, is the most widely deployed version. It enables multiple clients to access the same mailbox concurrently and supports complex mailbox structures, searches, and selective message retrieval. Core commands include SELECT, EXAMINE, FETCH, STORE, SEARCH, and IDLE. The protocol is extensible, with many extensions addressing quotas, ACLs, annotations, and other capabilities to enhance server interaction and administration.

Security and authentication considerations are central to IMAP use. IMAP can operate over TLS, using port 993 for implicit TLS or port 143 with STARTTLS. Authentication typically utilizes SASL, with common methods such as PLAIN, LOGIN, CRAM-MD5, and more modern options like SCRAM-SHA-1 or OAuth2. Many providers require encrypted connections and may implement additional authentication controls or multi-factor requirements.

In practice, IMAP is well suited for users who access mail from multiple devices or need robust server-side organization and search. It can be more bandwidth-intensive than POP when large portions of mailbox data are synchronized, though it preserves message state and enables real-time updates through features like IDLE. IMAP is supported by virtually all major email clients and servers.