Sendmail - Components of an Electronic Mail System

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Components of an Electronic Mail System

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 website at There are also commercially available Mail User Agents.

MUA 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.

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:


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.

Note: 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 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

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.


  • RFC 1521 - MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies



  • Alejandro Marin Badilla, information is an abridge version of Chapter 7 - Configuring Mail, from the System Administrator's Guide for HP-UX 11i v3.

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