Internet Message Access Protocol and Post Office Protocol

AIX® provides two Internet-based mail protocol server implementations for accessing mail remotely.

  • Post Office Protocol (POP or POP3DS)
  • Internet Message Access Protocol (IMAP or IMAPDS)

Each type of server stores and provides access to electronic messages. Using these mail access protocols on a server eliminates the requirement that, to receive mail, a computer must always be up and running.

The POP or POP3DS server provides an offline mail system, whereby a client, using POP or POP3DS client software, can remotely access a mail server to retrieve mail messages. The client can either download the mail messages and immediately delete the messages from the server, or download the messages and leave the messages resident on the POP or POP3DS server. After the mail is downloaded to the client machine, all mail processing is local to the client machine. The POP server allows access to a user mailbox one client at a time. The POP3DS version uses the OpenSSL libraries, which require security certificates.

The IMAP or IMAPDS server provides a superset of POP functionality but has a different interface. The IMAP or IMAPDS server provides an offline service, as well as an online service and a disconnected service. The protocol is designed to permit manipulation of remote mailboxes as if they were local. For example, clients can perform searches and mark messages with status flags such as deleted or answered. In addition, messages can remain in the server database until explicitly removed. The IMAP server also allows simultaneous interactive access to user mailboxes by multiple clients. The IMAPDS version uses the OpenSSL libraries, which require security certificates.

Each type of server is used for mail access only. These servers rely on the Simple Mail Transfer Protocol (SMTP) for sending mail.

Each protocol is an open protocol, based on standards described in RFCs. The IMAP servers are based on RFC 2060 and 2061, and the POP servers are based on RFC 1939. Both are connection-oriented using TCP sockets. The IMAP server listens on port 143, and the IMAPDS server listens on port 993. The POP server listens on port 110, and the POP3DS server listens on port 995. All servers are handled by the inetd daemon.

Requirement: To use the OpenSSL versions, you must install OpenSSL. OpenSSL is available on the AIX Toolbox for Linux Applications CD.