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A primer on messaging standards: SMTP, POP and IMAP

A brief introduction to three of Exchange's more commonly used messaging standards: SMTP, POP and IMAP.


No Exchange Server is an island. Exchange has to communicate with other Exchange servers and Internet mail servers that are not running Exchange. To make that possible, Exchange utilizes various messaging standards. In this article, I give you a brief introduction to three of Exchange's more commonly used messaging standards: SMTP, POP and IMAP.


Simple Message Transfer Protocol is probably the most important protocol for messaging. Before SMTP was designed, e-mail messages were sent using File Transfer Protocol (FTP). The sender would compose the message and then transmit it to the recipient in the form of a file. Although this technique worked pretty well, it had its shortcomings. Since the FTP protocol was designed to transmit files, not messages, it did not provide a way to identify the sender or the intended recipient. If a message were to show up on a company's FTP server, the administrator would have to open or print the message before they would even know who it was intended for.

The SMTP protocol was designed so that sender and recipient information could be transmitted along with the message. The design process didn't happen overnight though. The SMTP protocol has evolved a lot over the years. It was initially defined in 1973 by RFC 561. Since then, it has been modified by RFC 680, 724 and 733. The current RFCs applying to SMTP are 821 and 822.

SMTP is a two-way protocol that functions over TCP port 25. A lot of people don't realize it, but SMTP can be used to both send and receive messages. Typically though, workstations use POP rather than SMTP to receive messages. Instead, SMTP is usually used for either sending a message from a workstation to a mail server or for communications between mail servers. Exchange Server 2003, for example, uses SMTP for inter-server communications.

SMTP is a fairly complex protocol with numerous RFC extensions. If you would like to read more detailed information about the SMTP protocol, you can do so here.


Although SMTP can be used to send and receive messages, using SMTP for this purpose is often impractical or impossible. In order to receive messages through SMTP, the client must have a constant connection to the host. An example of a situation in which this is impossible is a dial-up Internet account.

To get around SMTP's constant-connection requirement, the Post Office Protocol was introduced. POP is a small protocol whose only purpose is to download messages from a server. This means allows a server to store a client's messages until the client connects. Once the client connects, the POP protocol downloads the client's mail and then deletes it from the server (by default) to make room for more mail. If a user wishes to respond to a message that was downloaded using POP, they can do so using SMTP.

The POP protocol is defined by RFC 1939, and functions across TCP port 110. You can read more about this protocol here.


When messages are downloaded using POP, the messages are automatically deleted from the server once the download completes. The reason why this is such a problem is because if POP were the only standard for downloading messages, users would have to save their messages locally. This presents challenges with backing up messages. It also becomes a problem if users move from computer to computer, because their messages do not move with them.

To get around these problems, Exchange Server uses a standard called Internet Messaging Access Protocol. IMAP allows messages to be kept on the server, but viewed and manipulated as though they were stored locally. IMAP is a part of the RFC 2060 specification, and functions over TCP port 143. You can read more about IMAP here.

About the author: Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. Brien has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer he has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at

Click here to read part two of this series, A primer on messaging standards: NNTP, X.400 and LDAP.



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