Port 143

Internet Message Access Protocol (IMAP), standardized primarily by RFC 3501, is a widely used protocol designed for retrieving and managing email messages stored on a mail server. Developed initially in the 1980s, it offers a sophisticated set of commands enabling clients to maintain complex folder hierarchies, perform searches, and download specific portions or attributes of messages, which reduces bandwidth consumption and improves responsiveness on limited connections.

IMAP operates primarily over TCP on port 143, enabling persistent bidirectional communication between the client and the server. A distinctive feature of IMAP compared to other email retrieval protocols such as POP3 is that it allows users to manipulate messages directly on the server rather than downloading and deleting them locally. This model supports consistent mailbox state across multiple devices, as users can read, flag, delete, or move messages with changes reflected globally. Clients often use IMAP in conjunction with SMTP (commonly on ports 25, 587, or 465) for outbound message submission.

The protocol supports advanced capabilities like folder management (create, delete, rename folders), message flag management (read/unread, answered, flagged), server-side searching, and partial message fetching. Especially in its modern version, IMAP4rev1, extensions enhance its capabilities, including support for idle notifications (push email), server-side sorting, quota management, and more. While IMAP natively transmits data in plaintext, it is generally paired with Transport Layer Security (TLS) to protect confidentiality and integrity during transit.