Email has become a critical tool in people's daily life and the email technologies have evolved from simple memo sending and receiving channels to complex and automated messaging systems.
There are several different technologies and approaches to building a distributed electronic mail infrastructurePOP (Post Office Protocol), SMTP (Simple Mail Transfer Protocol) and IMAP (Internet Message Access Protocol) among them. Very often, an Email system may support multiple technologies.
POP was designed to support "offline" mail processing, in which mail is delivered to a server and a personal computer user periodically invokes a mail "client" program that connects to the server and downloads all of the pending mail to the user's own machine. The offline access mode is a kind of store-and-forward service, intended to move mail (on demand) from the mail server (drop point) to a single destination machine, usually a PC or Mac. Once delivered to the PC or Mac, the messages are then deleted from the mail server. POP3 is the current version of POP. It is not designed to provide extensive manipulation operations of mail on the server; which are done by a more advanced (and complex) protocol IMAP4. POP3 uses TCP as the transport protocol.
Simple Mail Transfer Protocol (SMTP) is a protocol designed to transfer electronic mail reliably and efficiently. SMTP is a mail service modeled on the FTP file transfer service. SMTP transfers mail messages between systems and provides notification regarding incoming mail.
SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. An important feature of SMTP is its capability to transport mail across networks, usually referred to as "SMTP mail relaying". A network consists of the mutually TCP-accessible hosts on the public Internet, the mutually TCP-accessible hosts on a firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN environment utilizing a non-TCP transport-level protocol. Using SMTP, a process can transfer mail to another process on the same network or to some other network via a relay or gateway process accessible to both networks.
Internet Message Access Protocol (IMAP) is a method of accessing electronic mail or bulletin board messages that are kept on a mail server. IMAP permits a "client" email program to access remote message stores as if they were local. Email stored on an IMAP server can be manipulated from a desktop computer remotely, without the need to transfer messages or files back and forth between these computers.
In the online mode, the IMAP mail client does not copy mails in a shared server all at once and then delete them. It is an interactive client-server model, where the client can ask the server for headers or the bodies of specified messages or to search for messages meeting certain criteria. Messages in the mail repository can be marked with various status flags (e.g. "deleted" or "answered") and they stay in the repository until explicitly removed by the user. IMAP is designed to permit manipulation of remote mailboxes as if they were local. Depending on the IMAP client implementation and the mail architecture desired by the system manager, the user may save messages directly on the client machine or save them on the server or be given the choice of doing either.
MIME, an acronym for Multipurpose Internet Mail Extensions, specifies how messages must be formatted so that they can be exchanged between different email systems. MIME is a very flexible format, permitting one to include virtually any type of file or document in an email message. MIME messages can contain text, images, audio, video, or other application-specific data.
A secure version of MIME, S/MIME (Secure/Multipurpose Internet Mail Extensions), is defined to support encryption of email messages. Based on the MIME standard, S/MIME provides the following cryptographic security services for electronic messaging applications: authentication, message integrity and non-repudiation of origin and privacy and data security.
Email Technologies Overview
Related Terms: LAN, MAN, Packet Switching, Circuit Switching, Frame Relay, ATM, PPP, X.25, IP, SONET/SDH, VPN, mesh topology