IMAP (Internet Message Access Protocol) is a standard protocol for accessing and managing email messages stored on a mail server. Unlike POP3, it keeps messages on the server and synchronizes state such as folders and read/unread flags across multiple clients.
IMAP
IMAP, the Internet Message Access Protocol, is the long-standing standard for reading and managing email that lives on a mail server rather than on a single device. Where POP3 downloads and removes messages, IMAP keeps them on the server and synchronizes state — folders, flags, read/unread status — so the same mailbox looks consistent across a phone, a laptop, and a webmail client. The current revision, IMAP4rev2, is specified in RFC 9051.
- Server-side mailboxes - Messages and folders stay on the server, making the mailbox the source of truth rather than one client.
- Multi-client synchronization - Flags and folder structure stay in sync across every device connected to the account.
- Selective fetching - Clients can retrieve headers, a single part, or a full message, which is efficient over slow connections.
- Stateful sessions - A connection selects a mailbox and issues commands (
SELECT,FETCH,STORE,SEARCH) within that context.
In API terms, IMAP is the protocol that email-centric platforms wrap when they expose modern REST or webhook APIs over a mailbox — providers sync IMAP accounts server-side and hand developers a cleaner interface for reading and searching mail. It remains the backbone of email interoperability, and understanding it is still essential whenever an API’s job is to reach into a user’s inbox rather than reinvent messaging from scratch.
