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Lesson 4Responsibilities of sendmail
ObjectiveList the major responsibilities of the sendmail agent.

Responsibilities of sendmail Agent

Relaying (Forwarding)

The principal task of the sendmail program is to function as a Mail Transfer Agent, forwarding email to its correct destination. On a typical small network, one machine is configured to act as a mail hub.
Suppose that this machine is named HUB and that another machine on the network is named CLIENT. Mail from a user on CLIENT to a remote site takes the following path:
  1. The user’s MUA on CLIENT contacts the sendmail program on CLIENT and passes it the email message.
  2. The sendmail on CLIENT contacts the sendmail on HUB using a TCP connection to the SMTP port (25), then passes the mail to HUB.
  3. HUB contacts a remote mail-relaying site and passes the email message on. Eventually it reaches its destination.
Similarly, incoming mail is delivered to HUB, which is responsible for routing the mail properly on the local network.
Not all email comes from people. Many UNIX services, such as crond, use electronic mail to send alert messages. The daemon programs supplying such services contact sendmail directly to forward their messages.


In the process of forwarding email, sendmail (and other MTA’s) attaches a series of headers to the top of the message. These headers contain information such as the path the message followed to its destination, the sender and intended recipient of the message, and the date and time the message was sent.
For example, the FROM: header specifies the sender of the message, and may be used by the recipient to construct a reply. Because the headers are generated by the sendmail program, it can adjust the FROM: header so that replies to the message are sent to a specific place.


The process of manipulating headers plays an important role in masquerading. Suppose that a user named james is logged on to the machine CLIENT, as in the example at the beginning of this lesson. Suppose further that CLIENT is part of the domain corporation.com. Email from james on the machine CLIENT is in fact from [email protected], but user james prefers that the mail appear to be from his “real” email address, which is [email protected]. The sendmail program can be configured to rewrite the “FROM:” header on james’s messages so that they appear to be from [email protected]. This process is called masquerading.


The sendmail agent and other MTAs can also maintain a database of address aliases. An alias is simply an alternative address for a specific user. Aliasing allows all users to refer to users by simple nicknames or by names other than their UNIX username. Aliasing is different from masquerading, because it works with sendmail on the local host. Unlike masquerading, aliasing does not rewrite message headers to change the machine name. Perhaps the best way to explain aliasing is to provide an example:

sjstanger: [email protected]
djstanger: [email protected]
drstanger: [email protected]
sstanger: [email protected]
jstanger: [email protected]
jastanger: [email protected]

Principal Task

  1. Stay Organized. Principal Task tracks all of the dimensions of your todos so that you can organize things your way.
  2. Never Miss a Deadline. Keep track of important dates, recurring tasks, and get reminders so you always complete your todos on time
  3. Maintain momentum.
Each line constitutes an alias. First is the username, then the assigned email name that SMTP uses.

Sendmail Responsibilities - Quiz

Click the Quiz link below to take a brief multiple-choice quiz on sendmail responsibilities.
Sendmail Responsibilities - Quiz