IMAP4rev1

IMAP4rev1 is the revision one of the Internet Message Access Protocol (IMAP) used for accessing email on a mail server. It was introduced in 1994 as part of RFC 2177 and became the de‑facto standard for email retrieval and management, superseding the earlier IMAP2 protocol. The protocol enables clients to access and manipulate messages stored on the server without locally downloading them, supporting features such as mail folder hierarchies, message flags, search, and incremental updates.

The primary design goals of IMAP4rev1 were scalability, flexibility, and efficiency. It uses a client–server model over TCP, normally on ports 143 (clear text) and 993 (secure SSL/TLS). Commands are sent as simple ASCII strings and responses are structured in a tree format that allows the server to represent complex data like flags, message bodies, and search results. The protocol also defines various capabilities, including extended message syntax, message synchronization, and support for multiple concurrent sessions from different clients.

Security and privacy measures in IMAP4rev1 include optional authentication methods (CRAM‑MD5, DIGEST‑MD5, and OAuth2) and Transport Layer Security for encrypted connections. The protocol also supports access control lists and the ability to lock messages to prevent simultaneous modifications. Widely supported by mail clients such as Mozilla Thunderbird, Apple Mail, and Microsoft Outlook, IMAP4rev1 remains the foundation of most modern email systems, providing robust and efficient mailbox access across distributed environments.