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 pipe mail through other programs. It can queue mail for later delivery and understand the aliasing of one recipient name to another.
The sendmail program’s role (position) in the local filesystem hierarchy can be viewed as an inverted tree (see Figure 1-3).
When sendmail is run, it first reads the /etc/mail/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:
grep =/ /etc/mail/sendmail.cf
The output produced by the grep(1) command might look something like the following:
O AliasFile=/etc/mail/aliases #O ErrorHeader=/etc/mail/error-header O HelpFile=/etc/mail/helpfile O QueueDirectory=/var/spool/mqueues/q.* O StatusFile=/etc/mail/statistics #O UserDatabaseSpec=/etc/mail/userdb #O ServiceSwitchFile=/etc/mail/service.switch #O HostsFile=/etc/hosts #O SafeFileEnvironment=/arch #O DeadLetterDrop=/var/tmp/dead.letter O ControlSocketName=/var/spool/mqueues/.control #O PidFile=/var/run/sendmail.pid #O DefaultAuthInfo=/etc/mail/default-auth-info Mlocal, P=/usr/lib/mail.local, F=lsDFMAw5:/|@qPSXfmnz9, S=EnvFromSMTP/HdrFromL, Mprog, P=/bin/sh, F=lsDFMoqeu9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, D=$z:/,
Notice that some lines begin with an
O character, some with an
and others with a
marks a line as a configuration option. The word following the
O is the name of the option. The options in the
preceding output show the location of the files that
example, defines the location of the aliases(5)
database. The lines that begin with
delivery agents. The lines that begin with a
# are comments.
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 here:
# Mandatory aliases. postmaster: bob MAILER-DAEMON: postmaster abuse: postmaster # The five forms of aliases John_Adams: adamj xpres: ford,carter,reagan,bush,clinton oldlist: :include:/usr/local/oldguys nobody: /dev/null ftphelp: |/usr/local/bin/sendhelp
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
are ignored. As the first comment indicates, three aliases are
mandatory in every aliases file. They 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
all the same.
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 might not be.) When
sendmail is processing the envelope, and when it
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 : character. For example, the
postmaster becomes the new
After a name is substituted, the new name is then looked up, and the
process is repeated until no more matches are found. The name
MAILER-DAEMON is first
postmaster is looked up again and changed to
bob. Because there is no entry for
bob in the aliases file, the
mail message is delivered into
Every aliases file must have an alias for
postmaster 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.
When mail is bounced (returned because it
could not be delivered), it is always sent from
MAILER-DAEMON. That alias is needed because users
might reply to bounced mail. Without it, replies to bounced mail
would themselves bounce.
The five types of lines in an aliases file are as follows:
John_Adams: adamj xpres: ford,carter,reagan,bush,clinton oldlist: :include:/usr/local/oldguys nobody: /dev/null ftphelp: |/usr/local/bin/sendhelp
You have already seen the first line (it was the form used to convert
bob). In the
previous example, mail sent to
delivered to the user whose login name is
xpres: line shows how one name can be expanded
into a list of many names. Each new name 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
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
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 /usr/local/bin/sendhelp).
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 12.
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 a message is eventually delivered, sendmail stores it in a queue directory until the message 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
Beginning with V8.10, sendmail allows multiple queue directories to be used. Such a declaration can look like this:
Here, sendmail will use the subdirectories in /var/spool/queues that begin with the name q. for storage of messages. Such directories might be called, for example, q.00 and q.01.
If you have permission, take a look at a sendmail queue directory. It might be empty if no mail is waiting to be sent. If it is not empty, it will contain files such as these:
dfg17NVhbh002596 dfg1BHotav010793 qfg17NVhbh002596 qfg1BHotav010793
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
qf. The body of the mail message is saved in a
file whose name begins with the characters
The previous example shows two queued mail messages. One is
identified by the unique string
the other by
The internals of the queue files and the processing of those files are covered in Chapter 11.
Another role of the sendmail program is to deliver mail messages to local users. A local user is one who has a mailbox on the local filesystem. Delivering local mail is done by appending a message to the user’s mailbox, by feeding the mail message to a program, or by appending the message to a file other than the user’s mailbox.
In general, sendmail does not put mail messages directly into files. You saw the exception in the aliases file, in which you could specifically tell sendmail to append mail to a file. This is the exception, not the rule. Usually, sendmail calls other programs to perform delivery. Those other programs are called delivery agents.
In your sendmail.cf file you found two lines that defined local delivery agents, the ones that sendmail uses to deliver mail to the local filesystem:
Mlocal, P=/usr/lib/mail.local, F=lsDFMAw5:/|@qPSXfmnz9, S=EnvFromSMTP/HdrFromL, Mprog, P=/bin/sh, F=lsDFMoqeu9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, D=$z:/,
The /usr/lib/mail.local program is used to append mail to the user’s mailbox. The /bin/sh program is used to run other programs that handle delivery.
The configuration file line that begins with
Mlocal defines how mail is appended to a
user’s mailbox file. That program is usually
/usr/lib/mail.local (or with older systems,
/bin/mail) but can easily be a program such as
deliver(1) or procmail(1).
a user’s mailbox is a single file that contains a
series of mail messages. The usual Unix convention (but not the only
possibility) is that each message in a mailbox begins with a line
that starts with the five characters
From " (the
fifth is a blank space) and ends with a blank line.
The sendmail program neither knows nor cares
what a user’s mailbox looks like. All it cares about
is the name of the program that it must run to add mail messages to
that mailbox. In the example, that program is
configuration lines that define delivery agents are covered in detail
in Chapter 20.
Here, mail sent to the address
transformed via an alias into the new address
character at the start of this new address tells
sendmail that this is a program to run rather
than a file to append to. The intention here is that the program will
receive the mail and do something useful with it.
The sendmail program doesn’t
run mail delivery programs directly. Instead, it runs a shell and
tells that shell to run the program. The name of the shell is listed
in the configuration file in a line that begins with
Mprog, P=/bin/sh, F=lsDFMoqeu9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, D=$z:/,
In this example the shell is the /bin/sh(1). Other programs can appear in this line, such as /bin/ksh(1), the Korn Shell, or smrsh(1), the sendmail restricted shell that is supplied with the source distribution.
Another role of sendmail is that of transporting mail to other machines. A message is transported when sendmail determines that the recipient is not local. The following lines from a typical configuration file define delivery agents for transporting mail to other machines:
Msmtp, P=[IPC], F=mDFMuX, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP/HdrFromSMTP, Muucp, P=/usr/bin/uux, F=DFMhuUd, S=FromU, R=EnvToU/HdrToU, M=100000,
The actual lines in your file might differ. The name
smtp in the preceding example might appear in your
something else. The name
uucp might appear as
uucp-dom. There might
be more such lines than we’ve shown here. The
important point for now is that some delivery agents deal with local
delivery, while others deal with delivery over a network.
Msmtp, P=[IPC], F=mDFMuX, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP/HdrFromSMTP,
[IPC] might appear as
[TCP], but note that, beginning with V8.10
sendmail, the expression
[TCP] is deprecated, and it has been dropped
entirely in V8.12.
When sendmail transports mail on a TCP/IP network, it first sends the envelope-sender’s address to the other site. If the other site accepts the sender’s address as legal, the local sendmail then sends the list of envelope-recipient addresses. The other site accepts or rejects each recipient address one by one. If any recipient addresses are accepted, the local sendmail sends the message (header and body together). This kind of transaction for sending email is called SMTP and is defined in RFC2821.
UUCP is an old-style means of moving email between machines that are only connected with dial-up modems. The line in the configuration file that tells sendmail how to transport over UUCP might look, in part, like this:
Muucp, P=/usr/bin/uux, F=DFMhuUd, S=12, R=22/42, M=10000000,
This line tells sendmail to send UUCP network mail by running the /usr/bin/uux (UNIX to UNIX eXecute) program.
sendmail can use many other kinds of network protocols to transport email. Some of them might have shown up when you ran grep earlier. Other common possibilities might look, in part, like one of these:
Mfax, P=/usr/local/bin/faxmail, F=DFMhu, S=14, R=24, M=100000, Mmail11, P=/usr/etc/mail11, F=nsFx, S=Mail11From, R=Mail11To, Mmac, P=/usr/bin/macmail, F=CDFMmpsu, S=MailMacFrom, R=MailMacTo, A=macmail -t $u
Mfax line defines one of the many possible
ways to send a fax using
sendmail. A fax machine transports images of
documents over telephone lines. In the preceding configuration line,
the /usr/local/bin/faxmail program is run, and a
mail message is fed to it for conversion to and transmission as a fax
In all these examples, note that sendmail sends email over other networks by running programs that are tailored specifically for that use. Remember that the only network sendmail can use directly is a TCP/IP-based network.
Just as sendmail can transport mail messages over a TCP/IP-based network, it can also receive mail that is sent to it over the network. To do this, it must be run in daemon mode. A daemon is a program that runs in the background independent of terminal control.
As a daemon, sendmail is started once, usually when your machine is booted. Whenever an email message is sent to your machine, the sending machine talks to the sendmail daemon that is listening on your machine:
grep sendmail /etc/rc*←BSD-based systems %
grep sendmail /etc/init.d/*←SysV-based systems %
grep sendmail /etc/*rc←HP-UX systems (prior to HP-UX 10.0)
One typical example of what you will find is:
/etc/rc.local:if [ -f /usr/lib/sendmail -a -f /etc/mail/sendmail.cf ]; then /etc/rc.local: /usr/lib/sendmail -bd -q1h; echo -n ' sendmail'
The second line in this example shows that sendmail is run at boot time with a command line of:
/usr/lib/sendmail -bd -q1h
-bd command-line switch tells
sendmail to run in daemon mode. The
-q1h command-line switch tells
sendmail to wake up once per hour and process
the queue. Command-line switches are covered in Chapter 15.
 If you are not currently running sendmail V8.7 or above, you will have to grep(1) 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.
 Lines that begin
K might also appear.
If so, ignore them for now.
 According to RFC2822, all usernames
are case-sensitive except
postmaster. And RFC2142 defines additional names,
abuse, that are
not case-sensitive. But
sendmail, when processing its aliases file,
normally views all other names as case-insensitive too, if
F=u (F=u) is set on the
local delivery agent.
 The name
required by RFC2822, so resist the temptation to redefine it as
 Although for historical reasons, the sendmail developers still continue to use the term “mailers.”
 Actually, delivery agent definitions often span multiple lines.
 Actually, we’re fudging for simplicity. V8 sendmail can also send messages over an ISO network.