Chapter 5

How Intranet E-Mail Works

• How E-Mail Is Delivered within an Intranet
• How E-Mail is Delivered among Intranets

Intranets allow people to work together better by allowing them to communicate better. The most time-honored-and still the most popular-means of communication using computers is e-mail. Using e-mail, people can send messages to anyone else on the intranet-and, in fact, to anyone connected to the Internet as well, or connected to a computer network that has a connection to the Internet, such as an online service.

Intranet e-mail uses the TCP/IP protocol. The TCP protocol breaks your messages into packets, the IP protocol delivers the packets to the proper location, and then TCP reassembles the message on the receiving end so that it can be read.

You can also attach binary files, such as pictures, videos, sound, and executable files, to your e-mail messages. Since the Internet can't directly handle binary files in e-mail, the file must first be encoded in one of a variety of encoding schemes. Popular schemes are Base64 and uuencode. The person who receives the attached binary file must decode the file with the same scheme that was used to encode the file. Many e-mail software packages do this automatically.

When e-mail formats differ (and they often do, having developed from proprietary and incompatible products), gateways are used to translate the data into the appropriate format for the recipient. However, several standards have been developed that have been adopted by most e-mail companies today that allow the various products to communicate with each other. The Simple Mail Transfer Protocol (SMTP) and the X.400 MHS (message handling service) are the two most common protocols.

SMTP is based on a client/server model in which someone uses a mail client to create mail and read mail, while servers do the actual processing and delivery of the mail.

To create e-mail, you use a mail client, which is called a mail user agent or user agent (MUA or UA). There are multiple kinds and different agents for different types of computers. When mail is sent, the Message Transfer System (MTS) on a server uses a mail transfer agent (MTA) to examine the address of the person to which the mail is being sent. If the person can be found on the intranet, the mail is delivered to a mail delivery agent (MDA). The MDA then delivers the mail to the intended recipient. When you send e-mail to someone on another intranet or on the Internet, the message is instead sent by the MTA through the Internet. The message often has to travel through a series of networks before it reaches the recipient-networks that might use different e-mail formats.

Gateways are not attached to one particular machine or one combination of hardware and software, nor are they restricted to e-mail processing functionality. They can fulfill a variety of roles in addition to protocol conversion. One example of this is actually translating data from one format to another, as they do for PC to mainframe connectivity. Gateways differ from linking hardware such as bridges and routers by operating at higher OSI levels, although routers, because of their protocol conversion functionality, are also commonly called gateways.

How E-Mail Is Delivered within an Intranet

Probably the most heavily used part of an intranet has nothing to do with corporate databases, flashy Web pages, or multimedia content-it's the use of electronic mail. Corporate intranets can use a number of different e-mail programs, such as cc:Mail, Microsoft Mail, or Lotus Notes, among others. But the most common architecture underlying the use of intranet e-mail is the protocol called the Simple Mail Transfer Protocol, or SMTP. Illustrated here is how SMTP is used to deliver mail within an intranet.

1. As is true with many intranet and Internet applications, SMTP uses client/server architecture. When someone wants to create a mail message, they use a mail user agent or user agent (MUA or UA)-client software that runs on a computer-to create a piece of electronic mail. This MUA can be one of any number of e-mail programs, and can run on a variety of different computers, including PCs, Macintoshes, and UNIX workstations. Popular MUAs include Pine for UNIX computers; Pegasus, Eudora, cc:Mail, and Microsoft Mail for the PC; and Eudora for the Macintosh.
2. After the message is completed, it is sent by the MUA to a program running on a server called a mail transfer agent (MTA). The MTA looks at the address of the intended recipient of the message. If the recipient of the message is on the intranet, the MTA sends the message to another server program on the intranet called a mail delivery agent (MDA). If, instead, the recipient is located on the Internet or another intranet, the file is sent over the Internet to the recipient. (See the next illustration on how mail is transferred among intranets.) The MDA looks at the address of the recipient, and sends the mail to the in box of the proper person.
3. Some mail systems use another e-mail protocol called the Post Office Protocol (POP) in concert with SMTP. With POP, e-mail is not delivered directly to your computer. Instead, the mail is delivered to a mailbox on a server. To get the mail, someone logs onto the server using a password and user name, and retrieves mail with their mail agent.
4. The recipient of the mail can now use a mail user agent to read the mail, file it, and respond to it.
5. SMTP can only handle the e-mail transfer of plain ASCII text files. In order to send binary files such as spreadsheets, pictures, and word processing documents, they must first be converted into an ASCII format by encoding them. The files can be encoded using a variety of methods, including uuencoding and Base64. Some e-mail software will automatically encode binary files. When an encoded file is received by someone, they decode it and then can use or view the binary file. Again, many e-mail packages automatically decode encoded files.

How E-Mail is Delivered among Intranets

Often, e-mail created on an intranet will not be delivered to a computer on the intranet, but instead to someone on the Internet, to another intranet, or to an online service such as America Online, the Microsoft Network, or CompuServe. Here are the steps a typical message might take when being delivered from an intranet to another network or intranet.

1. An e-mail message is created using SMTP, as described on the previous page. As with all information sent across the Internet, the message is broken up by the Internet's TCP protocol into IP packets. The address is examined by the intranet's mail transfer agent. If the address is found on another network, the mail transfer agent will send the mail across the intranet via routers to the mail transfer agent on the receiving network.
2. Before the mail can be sent out across the Internet, it first might have to pass through an intranet firewall-a computer that shields the intranet so that intruders cannot break into the network. The firewall keeps track of messages and data going into and out of the intranet. It keeps a record of traffic so that any security breaches can be tracked down.
3. The message leaves the intranet and is sent to an Internet router. The router examines the address and determines where the message should be sent, and then sends the message on the way.
4. The receiving network gets the e-mail message. A gateway there uses TCP to reconstruct the IP packets into a full message. The gateway then translates the message into the particular protocol the target network uses (such as CompuServe's mail format), and sends it on its way. The message may be required to also pass through a firewall on the receiving network.
5. The receiving network examines the e-mail address and sends the message to the specific mailbox where the message was intended to go, or uses the Post Office Protocol (POP) to deliver it to a mail server.
6. Gateways can actually change data (if needed) for connectivity. For e-mail it may convert CompuServe protocol to SMTP. Gateways are also used to connect PC's to IBM mainframes for example, ASCII to EBCDIC.