Internet Message Access Protocol — Version 4rev1
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M. Crispin · 2003-03
Abstract
The Internet Message Access Protocol version 4rev1 (IMAP4rev1) defines a protocol that allows clients to access and manipulate electronic mail messages on a remote server. Unlike POP3, IMAP maintains messages on the server, supporting folders, flags, server-side search, and partial message fetching. IMAP operates over TCP port 143 (or 993 with TLS).
Why This RFC Matters
IMAP transformed email from a download-and-delete model into a synchronized, server-centric experience. This shift proved essential as users began accessing email from multiple devices — desktops, laptops, and eventually smartphones. The ability to maintain folder hierarchies, mark messages as read/flagged, and search server-side without downloading all messages made IMAP the dominant protocol for webmail services and mobile email clients. Virtually all modern email clients and providers (Gmail, Outlook, Yahoo Mail) expose IMAP access, making RFC 3501 one of the most widely implemented email protocols.