The sendmail program plays a variety of roles, all critical to the proper flow of electronic mail. It listens to the network for incoming mail, transports mail messages to other machines, and hands local mail to a local program for local delivery. It can append mail to files and can pipe mail through other programs. It can queue mail for later delivery and understands the aliasing of one recipient name to another.
The sendmail program's role (position) in the local file system hierarchy can be viewed as an inverted tree (see Figure 3.1).
When sendmail is run, it first reads the /etc/sendmail.cf configuration file. Among the many items contained in that file are the locations of all the other files and directories that sendmail needs.
Files and directories listed in sendmail.cf are usually specified as full pathnames for security (such as /var/spool/mqueue rather than mqueue). As the first step in our tour of those files, run the following command to gather a list of them: 
 If you are not currently running V8.7 or above sendmail, you will have to grep for "/[^0-9].*/" instead. If you're not running sendmail at all, you won't be able to do this, so for now just read along instead.
grep =/ /etc/sendmail.cf
The output produced by the grep(1) command may look something like the following: 
 Lines that begin with
Kmay also appear. If so, ignore them for now.
O AliasFile=/etc/aliases #O ErrorHeader=/etc/sendmail.oE O HelpFile=/usr/lib/sendmail.hf O QueueDirectory=/var/spool/mqueue O StatusFile=/etc/sendmail.st #O UserDatabaseSpec=/etc/userdb #O ServiceSwitchFile=/etc/service.switch #O HostsFile=/etc/hosts #O SafeFileEnvironment=/arch Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10/30, R=20/40, Mprog, P=/bin/sh, F=lsDFMoeu, S=10/30, R=20/40, D=$z:/,
Notice that some lines begin with an
O character, some
M, and others with a
O marks a line as a configuration option.
The word following the
O is the name of the option.
The options in the above output show the location of the files that
AliasFile, for example, defines the
location of the aliases(5) database.
The lines that begin with
M define delivery agents.
The lines that begin with a
# are comments.
First we will examine the files in the
lines. Then we will discuss local delivery and the files
M delivery agent lines.
Aliasing is the process of converting one recipient name into another. One use is to convert a generic name (such as root) into a real username. Another is to convert one name into a list of many names (for mailing lists).
Compare what you find in your aliases file to the brief example of an aliases file listed below:
# Mandatory aliases.
# The five forms of aliases
oldlist: :include: /usr/local/oldguys
Your aliases file is probably far more complex, but even so, note that the example shows all the possible forms of aliases.
Lines that begin with
# are comments. Empty lines
As the first comment indicates, there are two
aliases that are mandatory in every aliases file.
Both are the simplest form of alias: a name and what to change
that name into.
The name on the left of the
is changed into the name on the right.
Names are not case-sensitive. For example,
postmaster are all the same.
 According to RFC822, all usernames are case-sensitive except postmaster. But sendmail, when processing its aliases file, views all names as case-insensitive.
For every envelope that lists a local user as a recipient,
sendmail looks up that recipient's name in the aliases
file. (A local user is any address that would normally be delivered
on the local machine.
That is, postmaster is local, whereas
postmaster@remote may not be.)
When sendmail matches the recipient to one of the
names on the left of the aliases file, it replaces that
recipient name with the text to the right of the
For example, the recipient
postmaster becomes the recipient
After a name is substituted, the new name is then looked up,
and the process is repeated until
no more matches are found.
MAILER-DAEMON is first changed
is looked up again and changed to
there is no entry for
root in the aliases file,
the mail message is delivered into root's mailbox.
Every aliases file must have an alias for
that will expand to the name of a real user.
Mail about mail
problems is always sent to
postmaster both by mail-related
programs and by users who are having trouble sending mail.
 The name
postmasteris required by RFC822 and RFC1648, so resist the temptation to redefine it as
When mail is
bounced (returned because it could not be delivered),
it is always sent from
That alias is needed because users may reply to bounced mail.
Without it, replies to bounced mail would themselves bounce.
The five types of lines in an aliases file areas are as follows:
oldlist: :include: /usr/local/oldguys
You have already seen the first (it was the form used to convert
root). In the above example, mail sent
John_Adams is delivered to the user whose
login name is
xpres: line shows how one name can
be expanded into a list of many names. Each of those new names
becomes a new name for further alias processing.
If a name can't be further expanded, a copy of the mail
message is delivered to it.
oldlist: line shows how a mailing list can be read
from a file. The expression
:include: tells sendmail to read a specific file
and to use the names in that file as the list of recipients.
nobody: line shows how a name can be aliased
to a file.
The mail message is appended to the file.
The /dev/null file listed here is a special one. That
file is an empty hole into which the mail message simply vanishes.
ftphelp: line shows how a name can be replaced by
the name of a program. The
| character causes
sendmail to pipe the mail message through the program whose
full pathname follows (in this case, we specified the full pathname as
The aliases file can become very complex. It can be used to solve many special mail problems. The aliases file is covered in greater detail in Chapter 24, Aliases.
A mail message can be temporarily undeliverable for a wide variety of reasons, such as when a remote machine is down or has a temporary disk problem. To ensure that such messages are eventually delivered, sendmail stores them in its queue directory until they can be delivered successfully.
The location of that directory must be a full pathname. Its exact
location varies from vendor to vendor, but you
can always find it by looking for the
QueueDirectory option in your
If you have permission, take a look at the queue directory. It may be empty if there is no mail waiting to be sent. If it is not empty, it will contain files like these:
dfQAA07038 dfMAA08000 qfQAA07038 qfMAA08000
When a mail message is queued, it is split into two parts,
each part being saved in a separate file. The header information
is saved in a file whose name begins with the characters
The body of the mail message is saved in a file whose name begins
with the characters
The example above shows two queued mail messages. One
is identified by the unique string
QAA07038 and the other
The internals of the queue files and the processing of those files are covered in Chapter 23, The Queue.