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HP-UX System Administrator's Guide: Configuration Management: HP-UX 11i Version 3 > Chapter 7 Configuring Mail

Components of an Electronic Mail System


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To properly configure an electronic mail system, you need to know about the following components:

Mail User Agents

Mail User Agents are the programs that users run to send, and read e-mail. Mail User Agents that are shipped with HP-UX include dtmail, elm, mail, and mailx. Mozilla Thunderbird can be downloaded from the HP web site at http://www.hp.com . There are also commercially available Mail User Agents.

Although Mail User Agents appear to do all the work of transmitting and receiving e-mail, they are merely the visible part of the entire electronic mail system. Mail User Agents do not actually deliver the e-mail. Electronic mail delivery is handled by Mail Delivery Agents.

Mail User Agents:

  • Format outgoing messages with proper header information and (if necessary) encode the outgoing messages for use by Mail Delivery Agents in routing the messages.

  • Allow users to read, save, and delete incoming electronic mail messages.

  • Schedule MIME Applications (if necessary) to allow the user to experience nontextual information attached to incoming electronic mail; for example, viewing graphics files or video clips, or listening to audio data.

Mail Delivery Agents

Mail Delivery Agents form the core of the electronic mail system. These programs, usually running in the background, are responsible for routing, and delivering electronic mail. On HP-UX and other UNIX systems, the primary Mail Delivery Agent is sendmail.

Although sendmailcan be run directly from a shell command line to send a message, it is not usually used in this way. Mail User Agents are usually used as front ends to sendmail for sending mail.

Mail Delivery Agents:

  • Deliver mail to local users (users receiving e-mail on the computer that the Mail Delivery Agent is running on) by scheduling the /usr/bin/mail program or by forwarding the mail to users on local client machines.

  • Forward e-mail via the appropriate transport mechanism not intended for local users to other computers/networks for delivery. For example, UUCP mail would be sent on its way by scheduling (and passing the message to) the uux program.

  • Modify the format of the address information in message headers to accommodate the needs of the next computer or network in a message’s delivery path, and to accommodate the delivery method that is being used to route the message. For example:

    UUCP addresses are of the form:


    whereas TCP/IP addresses can take one of several forms, such as:

    user user@computer user@computer.domain.name

Mail Alias Files

Mail Alias Files are used for:

  • Mapping “real world” names to user login names

  • Describing distribution lists (mailing lists), where a single name (for example, deptXYZ) is mapped to several or many user login names

For faster access, the alias files can be processed into a hashed database with the command newalias (a form of sendmail). By default, the system alias file (ASCII version) is located in the file /etc/mail/aliases.

The Mail Queue

Outgoing messages cannot always be sent right away because of down computers, broken network connections, network traffic, and other reasons. Your Mail Delivery Agent needs a place to hold these messages until they can be sent on their way. That place is the mail queue.

If you are using sendmail (supplied with HP-UX) as your Mail Delivery Agent, your mail queue is, by default, the directory /var/spool/mqueue.

Networking Topographies

Although there are many ways to configure electronic mail for a group of computers under your control, the following setups are often used:

  • Central Mail Hub

  • Gateway Mail Hub

  • Fully Distributed

Central Mail Hub

A central mail hub (a mail server) receives e-mail for its users and the users on the client computers that it serves. Users either NFS-mount their incoming mail files to their local computers (the clients), or log in to the hub to read their mail. Electronic mail can be sent directly from the client computers.

  • Only one computer needs to be connected to the outside world, which protects (hides) the local clients from the network outside, giving the appearance that all mail from the workgroup is coming from a central computer.

  • Only one computer needs to run the sendmail daemon (to “listen” for incoming e-mail).

  • Data are centralized (easier to backup and control)

  • Users of client machines must NFS-mount their incoming mail files from the hub (or log in to the hub) in order to read their mail.

  • All electronic mail, even between client machines in a local workgroup, must go through the hub computer. This means that local mail traffic could be slowed if the hub machine becomes overloaded; and mail traffic would stop completely if the hub goes down or becomes disconnected from the network.

Gateway Mail Hub

A gateway mail hub receives electronic mail for its users and users of client computers that it serves. The hub forwards mail intended for users of the client computers to those clients. Users do not NFS-mount their incoming mail files to their local (client) computers; they send and receive their mail directly from their own machines.

  • Only one computer needs to be connected to the outside world, which protects (hides) the local clients from the network outside, giving the appearance that all mail from the workgroup is coming from a central computer.

  • Traffic between local machines (within the workgroup) does not have to travel through the hub computer because each client can send and receive its own electronic mail. Therefore if the hub goes down or becomes overloaded, local mail traffic is unaffected (only mail to and from computers outside of the workgroup is affected).

  • Greater privacy for electronic mail users on the client machines. Data is not stored in a central repository.

  • Each computer needs to run its own copy of the sendmail daemon to “listen” for incoming mail.

  • Electronic mail from and to the outside world must travel through the hub, which could become a bottleneck if the mail traffic is heavy.

    If the hub is down, clients cannot send and receive mail to and from computers outside of the work group.

Fully Distributed

Each computer in the workgroup independently sends and receives its own electronic mail.

  • There is no hub computer to contend with in this setup. Every computer, whether local to the workgroup or not, can send and receive electronic mail directly with every other computer in the network that also supports electronic mail.

  • Greater privacy for electronic mail users on the individual machines. Data is not stored in a central repository.

  • Because each computer (from an electronic mail perspective) is connected directly to the outside world, there is an increased data security risk.

  • Each computer needs to run its own copy of the sendmail daemon to “listen” for incoming mail.

Selecting a Topography

The topography you use depends on your needs. Here are some things to consider when choosing your electronic mail network topography:


By using a topography with a hub computer you can better protect work that is being done on machines within your workgroup or organization. The single point of entry to your internal network (a gateway computer) is a lot easier to defend against unauthorized entry.

Data Centralization

By having your mail files on a single machine or directory structure, it is easier to back up your data.

Company Appearance and Future Planning

By using one of the topographies that use a hub computer, a small company can look more like a large corporation. As the company grows, the centralized mail processing can be easily moved to the jurisdiction of a corporate communications group.

Traffic Levels

If e-mail traffic levels are expected to be high, you might not want to use a single hub for processing all electronic mail.

MIME Applications

Gone are the days when electronic mail messages contained only ASCII text. Today people want to send other types of data: audio clips, still graphics (in a variety of formats), video clips, and so on.

Because Mail Delivery Agents were developed to handle the 7-bit ASCII data in text-only messages and not the 8-bit binary data contained in audio, graphics, and video, a method is needed for encoding the binary data to be transported by the text-only transport agents. The system developed for encoding the binary data is known as MIME (for Multipurpose Internet Mail Extensions).

Most modern Mail User Agents (including the CDE mail client, dtmail and the X-Window-enabled elm) can process MIME-encoded e-mail messages. For complete details about how MIME works, see RFC 1521. See also elm(1).

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