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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: computername@domain.name!username |
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: 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.
- Advantages:
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)
- Disadvantages:
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.
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. - Advantages:
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.
- Disadvantages:
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.
Each computer in the workgroup independently sends
and receives its own electronic mail. - Advantages:
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.
- Disadvantages:
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: - Security
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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