Internet Mail Access Protocol version 4 (IMAP4)

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Definition of Internet Mail Access Protocol version 4 (IMAP4) in Network Encyclopedia.

What is IMAP4 (Internet Mail Access Protocol version 4)?

IMAP4 stands for Internet Mail Access Protocol version 4, is an Internet standard protocol for storing and retrieving messages from Simple Mail Transfer Protocol (SMTP) hosts. Internet Mail Access Protocol version 4 (IMAP4) provides functions similar to Post Office Protocol version 3 (POP3), with additional features as described in this entry.

How it works

SMTP provides the underlying message transport mechanism for sending e-mail over the Internet, but it does not provide any facility for storing and retrieving messages. SMTP hosts must be continuously connected to one another, but most users do not have a dedicated connection to the Internet.


IMAP4 provides mechanisms for storing messages received by SMTP in a receptacle called a mailbox. An IMAP4 server stores messages received by each user until the user connects to download and read them using an IMAP4 client such as Microsoft Outlook 2000 or Microsoft Outlook Express.

IMAP4 includes a number of features that are not supported by POP3. Specifically, IMAP4 allows users to

  • Access multiple folders, including public folders
  • Create hierarchies of folders for storing messages
  • Leave messages on the server after reading them so that they can access the messages again from another location
  • Search a mailbox for a specific message to download
  • Flag messages as read
  • Selectively download portions of messages or attachments only
  • Review the headers of messages before downloading them

To retrieve a message from an IMAP4 server, an IMAP4 client first establishes a Transmission Control Protocol (TCP) session using TCP port 143. The client then identifies itself to the server and issues a series of IMAP4 commands:

  • LIST: Retrieves a list of folders in the client’s mailbox 
  • SELECT: Selects a particular folder to access its messages 
  • FETCH: Retrieves individual messages 
  • LOGOUT: Ends the IMAP4 session 


IMAP4 is supported by Microsoft Exchange Server. Because IMAP4 clients can allow read messages to remain on the IMAP4 server, IMAP4 is especially useful for mobile users who dial up and access their mail from multiple locations. The downside is that IMAP4 servers require more resources than POP3 servers because users tend to leave large numbers of messages on the server.


To troubleshoot problems with remote IMAP4 servers, use Telnet to connect to port 143. Then try issuing various IMAP4 commands such as the ones described in this entry and examine the results.