Internet Message Access Protocol (IMAP)

Key characteristics

• Server-side folders: Mailboxes and folders are maintained on the server so clients can browse and organize messages without permanently moving them to a local store.
• State synchronization: Read/unread flags, message flags, and folder membership are tracked on the server and reflected to all connected clients.
• Partial fetch: Clients can request headers or portions of a message (e.g., body preview or attachments) to save bandwidth and speed up access.
• Server search: Servers can perform searches and return matching messages, reducing the need to download entire mailboxes.
• Concurrent access: Multiple clients can access the same mailbox concurrently with predictable behavior.

IMAP is typically used alongside SMTP, which sends messages. While SMTP delivers mail to a server, IMAP is used to read and manage that mail. The most widely deployed version is often called IMAP4, and the protocol has been extended over time to add security and performance features.

How IMAP works in practice

When an email client connects, it authenticates, lists available mailboxes, and selects one to work with. Commands such as SELECT, FETCH, STORE and SEARCH are used to navigate mailboxes, retrieve message data, change flags, and rearrange messages. Because the server retains authoritative state, the client can cache data for offline use and reconcile changes when reconnecting. Many modern clients use secure transports (TLS) to protect credentials and message content in transit.

History, uses and distinctions

IMAP was developed to overcome limitations of download-only approaches so users could manage mail from multiple locations. It is common in environments where people check mail from phones, desktops, and web interfaces concurrently. In contrast, the Post Office Protocol (POP) was designed primarily to download and optionally remove messages from a server; POP is simpler but less suitable for multi-device workflows. For sending mail, SMTP remains the complementary protocol.

Security and extensions: IMAP supports authentication methods and is commonly run over encrypted channels (TLS). Extensions have added features such as server-side sorting, threading, and annotation. Administrators and client developers often consult protocol specifications and implementation guides for interoperability.

For further technical background and implementation details see protocol documentation, information about typical email clients, how a mail server stores mail, and comparisons with Post Office Protocol (POP).