IMAP does the same basic job as POP: it enables access to a mailbox stored on a remote server. For this reason, it's not too surprising that IMAP is somewhat similar to POP in operation, and you'll notice those similarities in this topic if you've already read about POP3. However, IMAP has many more features and capabilities than POP. Therefore, it's also not too surprising that IMAP would be more complex than POP in terms of how it operates, and I think you will notice this as well.

IMAP4 is a standard client/server protocol like POP3 and most other TCP/IP application protocols. For the protocol to function, an IMAP4 server must be operating on the server where user mailboxes are located. Again, as with POP3, this need not necessarily be the same physical server as the one that provides SMTP service. The mailbox must in some way be made accessible to both SMTP for incoming mail and to IMAP4 for message retrieval and modification. A mechanism for ensuring exclusive access to avoid interference between the various protocols is also needed.

IMAP4 uses the Transmission Control Protocol (TCP) for communication. This ensures that all commands and data are sent reliably and received in the correct order. IMAP4 servers listen on well-known port number 143 for incoming connection requests from IMAP4 clients. After a TCP connection is established, the IMAP4 session begins.