What RFC number defines the IMAP protocol?

RFC 3501 defines IMAP4rev1, the widely used revision of the Internet Message Access Protocol.

Published in March 2003, RFC 3501 specifies how an email client communicates with a server to access messages stored in remote mailboxes. Unlike POP3, which commonly downloads messages for local handling, IMAP keeps mail on the server and supports folders, flags, searches, partial retrieval, and synchronization across multiple devices.

IMAP4rev1 uses TCP and traditionally listens on port 143. Encrypted connections commonly use IMAPS on port 993, or STARTTLS to upgrade a connection on the standard IMAP port. The protocol is command-based: clients send tagged commands such as SELECT, FETCH, STORE, and SEARCH, and servers return tagged responses.

RFC 3501 is sometimes confused with other email standards. RFC 1939 defines POP3, RFC 5321 defines SMTP for sending and relaying mail, and RFC 2822 defines the format of Internet message headers and bodies. Newer IMAP extensions exist, but RFC 3501 remains the canonical specification for IMAP4rev1.