The encyclopedia · Software & IT · Technical decision · 1986–2003
IMAP kept mail on the server, so every device sees the same inbox
Mark Crispin's 1986 IMAP treats the mailbox as server-side state: clients manage folders remotely and stay in sync, so every device sees the same mail.
University of Washington
the move
Early email retrieval used POP, where clients downloaded messages and often deleted them from the server, so mail lived on whichever machine you checked. As people used multiple computers, their inboxes scattered.
Mark Crispin designed IMAP in 1986 as a remote access mailbox protocol: the mailbox stays on the server and clients manipulate folders, flags, and message parts remotely. RFC 3501, the current standard, describes access and manipulation functionally equivalent to local folders, with resynchronization for offline clients.
IMAP won because it matched how people actually worked: leave messages on the server, connect from anywhere, and multiple clients see the same state. Virtually all modern email clients and servers support it, and it is the protocol behind webmail and multi-device sync.
why it works
- Server-side state means no inbox is stranded on one device.
- Remote folder management matches how users organize mail.
- Partial fetching and server search work well on slow links.
- It was standardized as an IETF RFC, so every vendor implemented the same protocol.
what transfers
If users now own several devices, store the canonical state centrally and treat each device as a view; a protocol that assumes one machine loses as soon as people own two.
what came after
IMAP became the default mailbox protocol for multi-device email and the basis of webmail and push mail; it remains the standard alongside POP3 today.
references
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