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How sendmail works

How sendmail works

Outbound email :

1. MUA passes the email to sendmail , which creates in the /var/spool/mqueue (mail queue) directory two files that hold the message while sendmail processes it.
2. To create a unique filename for a particular piece of email, sendmail generates a random string and uses that string in filenames pertaining to the email.
3. The sendmail daemon stores the body of the message in a file named df (data file) followed by the generated string.
4. It stores the headers and other information in a file named qf (queue file) followed by the generated string.
5. If a delivery error occurs, sendmail creates a temporary copy of the message that it stores in a file whose name starts with tf (temporary file) and logs errors in a file whose name starts xf .
6. Once an email has been sent successfully, sendmail removes all files pertaining to that email from /var/spool/mqueue .

Incoming email :

1. By default, the MDA stores incoming messages in users’ files in the mail spool directory, /var/spool/mail , in mbox format. Within this directory, each user has a mail file named with the user’s username. Mail remains in these files until it is collected, typically by an MUA. Once an MUA collects the mail from the mail spool, the MUA stores the mail as directed by the user, usually in the user ‘s home directory hierarchy.

mbox versus maildir :

1. The mbox format stores all messages for a user in a single file. To prevent corruption, the file must be locked while a process is adding messages to or deleting messages from the file; you cannot delete a message at the same time the MTA is adding messages. A competing format, maildir , stores each message in a separate file. This format does not use locks, allowing an MUA to read and delete messages at the same time as new mail is delivered. In addition, the maildir format is better able to handle larger mailboxes

Mail logs :

# cat/var/log/maillog

Mar 3 16:25:33 MACHINENAME sendmail[7225]: i23GPXvm007224:
to=, ctladdr=
(0/0), delay=00:00:00, xdelay=00:00:00, mailer=local, pri=30514,
dsn=2.0.0, stat=Sent

Each log entry starts with a timestamp, the name of the system sending the email, the name of the mail server ( sendmail ), and a unique identification number. The address of the recipient follows the to= label and the address of the sender follows ctladdr= . Additional fields provide the name of the mailer and the time it took to send the message. If a message is sent correctly, the stat= label is followed by Sent .

Aliases and Forwarding :

Three files can forward email: .forward (page 634), aliases (discussed next ), and virtusertable (page 640). Table 20-1 on page 640 compares the three files.
Table 20-1. Comparison of forwarding techniques

.forward aliases virtusertable

Controlled by non root user root root

Forwards email
addressed to “non root user” “Any real or virtual user on the local system” “Any real or virtual user on any domain recognized by sendmail”

Order of precedence Third Second First


Most of the time when you send email, it goes to a specific person; the recipient, user@system , maps to a specific, real user on the specified system. Sometimes you may want email to go to a class of users and not to a specific recipient. Examples of classes of users include postmaster , webmaster , root , and tech_support . Different users may receive this email at different times or the email may be answered by a group of users. You can use the /etc/aliases file to map inbound addresses to local users, files, commands, and remote addresses.

Each line in /etc/aliases contains the name of a local pseudouser, followed by a colon , whitespace, and a comma-separated list of destinations. The default installation includes a number of aliases that redirect messages for certain pseudousers to root . These have the form

system: root

Sending messages to the root account is a good way of making them easy to review. However, because root ‘s email is rarely checked, you may want to send copies to a real user. The following line forwards mail sent to abuse on the local system to root and alex :

abuse: root, alex

You can create simple mailing lists with this type of alias. For example, the following alias sends copies of all email sent to admin on the local system to several users, including Zach, who is on a different system:

admin: sam, helen, mark,

You can direct email to a file by specifying an absolute pathname in place of a destination address. The following alias, which is quite popular among less conscientious system administrators, redirects email sent to complaints to /dev/null where they disappear:

complaints: /dev/null

You can also send email to standard input of a command by preceding the command with a pipe character ( | ). This technique is commonly used with mailing list software such as Mailman. For each list it maintains, Mailman has entries, such as the following entry for mylist , in the aliases file:

mylist: “|/usr/lib/mailman/mail/mailman post mylist”


After you edit /etc/aliases , you must either run newaliases as root or restart sendmail to recreate the aliases.db file that sendmail reads.


You can use praliases to list aliases currently loaded by sendmail :

# /usr/sbin/praliases| head-5


Systemwide aliases are useful in many cases, but non root users cannot make or change them. Sometimes you may want to forward your own mail: Maybe you want mail from several systems to go to one address or perhaps you just want to forward your mail while you are working at another office for a week. The ~/.forward file allows ordinary users to forward their email.

Lines in a .forward file are the same as the right column of the aliases file explained previously: Destinations are listed one per line and can be a local user, a remote email address, a filename, or a command preceded by a pipe character ( | ).

Mail that you forward does not go to your local mailbox. If you want to forward mail and keep a copy in your local mailbox, you must specify your local username preceded by a backslash to prevent an infinite loop. The following example sends Sam’s email to himself on the local system and on the system at :

$ cat ~sam/.forward

Related Programs


The sendmail package includes several programs. The primary program, sendmail , reads from standard input and sends an email to the recipient specified by its argument. You can use sendmail from the command line to check that the mail delivery system is working and to email the output of scripts.


The mailq utility displays the status of the outgoing mail queue and normally reports there are no messages in the queue. Messages in the queue usually indicate a problem with the local or remote sendmail configuration or a network problem.

# /usr/bin/mailq
/var/spool/mqueue is empty
Total requests: 0


The mailstats utility reports on the number and sizes of messages sendmail has sent and received since the date it displays on the first line:

# /usr/sbin/mailstats
Statistics from Sat Dec 24 16:02:34 2005
M msgsfr bytes_from msgsto bytes_to msgsrej msgsdis Mailer
0 0 0K 17181 103904K 0 0 prog
4 368386 4216614K 136456 1568314K 20616 0 esmtp
9 226151 26101362K 479025 12776528K 4590 0 local
T 594537 30317976K 632662 14448746K 25206 0
C 694638 499700 146185

In the preceding output, each mailer is identified by the first column, which displays the mailer number, and by the last column, which displays the name of the mailer. The second through fifth columns display the number and total sizes of messages sent and received by the mailer. The sixth and seventh columns display the number of messages rejected and discarded respectively. The row that starts with T lists the column totals, and the row that starts with C lists the number of TCP connections.

Setting Up a Backup Server

You can set up a backup mail server to hold email when the primary mail server experiences problems. For maximum coverage, the backup server should be on a different connection to the Internet from the primary server.

Setting up a backup server is easy. Just remove the leading dnl from the following line in the backup mail server’s file:

dnl FEATURE(‘relay_based_on_MX’)dnl

DNS MX records (page 726) specify where email for a domain should be sent. You can have multiple MX records for a domain, each pointing to a different mail server. When a domain has multiple MX records, each record usually has a different priority; the priority is specified by a two-digit number, where lower numbers specify higher priorities.

When attempting to deliver email, an MTA first tries to deliver email to the highest-priority server. If that delivery attempt fails, it tries to deliver to a lower-priority server. If you activate the relay_based_on_MX feature and point a low-priority MX record at a secondary mail server, the mail server will accept email for the domain. The mail server will then forward email to the server identified by the highest-priority MX record for the domain when that server becomes available.

Other Files in /etc/mail :

The /etc/mail directory holds most of the files that control sendmail . This section discusses three of those files: mailertable , access , and virtusertable .
mailertable : Forwards Email from One Domain to Another

When you run a mail server, you may want to send mail destined for one domain to a different location. The sendmail daemon uses the /etc/mail/mailertable file for this purpose. Each line in mailertable holds the name of a domain and a destination mailer separated by whitespace; when sendmail receives email for the specified domain, it forwards it to the mailer specified on the same line. Red Hat enables this feature by default: Put an entry in the mailertable file and restart sendmail to use it.

The following line in mailertable forwards email sent to to the mailer at :

$ cat /etc/mail/mailertable smtp:[]

The square brackets in the example instruct sendmail not to use MX records but rather to send email directly to the SMTP server. Without the brackets, email could enter an infinite loop.

A period in front of a domain name acts as a wildcard and causes the name to match any domain that ends in the specified name. For example, matches , , and so on.

The sendmail init script regenerates mailertable.db from mailertable each time you run it, as when you restart sendmail .
access : Sets Up a Relay Host

On a LAN, you may want to set up a single server to process outbound mail, keeping local mail inside the network. A system that processes outbound mail for other systems is called a relay host . The /etc/mail/access file specifies which systems the local server relays email for. As configured by Red Hat, this file lists only the local system:

$ cat /etc/mail/access

# by default we allow relaying from localhost…
localhost.localdomain RELAY
localhost RELAY RELAY

You can add systems to the list in access by adding an IP address followed by whitespace and the word RELAY . The following line adds the 192.168. subnet to the list of hosts that the local system relays mail for:

192.168. RELAY

The sendmail init script regenerates access.db from access each time you run it, as when you restart sendmail .
virtusertable : Serves Email to Multiple Domains

When the DNS MX records are set up properly, a single system can serve email to multiple domains. On a system that serves mail to many domains, you need a way to sort the incoming mail so that it goes to the right places. The virtusertable file can forward inbound email addressed to different domains ( aliases cannot do this).

As sendmail is configured by Red Hat, virtusertable is enabled. You need to put forwarding instructions in the /etc/mail/virtusertable file and restart sendmail to serve the specified domains. The virtusertable file is similar to the aliases file (page 633), except the left column contains full email addresses, not just local ones. Each line in virtusertable starts with the address that the email was sent to, followed by whitespace and the address sendmail will forward the email to. As with aliases , the destination can be a local user, an email address, a file, or a pipe symbol ( | ), followed by a command.

The following line from virtusertable forwards mail addressed to to zcs , a local user: zcs

You can also forward email for a user to a remote email address:

You can forward all email destined for a domain to another domain without specifying each user individually. To forward email for every user at to , specify as the first address on the line. When sendmail forwards email, it replaces the %1 in the destination address with the name of the recipient. The next line forwards all email addressed to to , keeping the original recipients’ names :

Finally you can specify that email intended for a specific user should be rejected by using the error namespace in the destination. The next example bounces email addressed to with the message 5.7.0:550 Invalid address : error:5.7.0:550 Invalid address

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