Mail transfer agent (MTA) basics

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Learn Linux, 101

Getting your mail through


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Content series:

This content is part # of # in the series: Learn Linux, 101

http://www.ibm.com/developerworks/library?series_title_by=Learn+Linux+101

Stay tuned for additional content in this series.

This content is part of the series:Learn Linux, 101

Stay tuned for additional content in this series.

Overview

In this tutorial, learn to:

  • Use the mail command.
  • Create mail aliases.
  • Configure email forwarding.
  • Understand common mail transfer agent (MTA) programs such as
    postfix,
    sendmail, qmail, and exim.

Controlling where your mail
goes

Email on a Linux system is delivered using MTAs. Your MTA delivers mail to other
users on your system and MTAs communicate with each other to deliver mail all over a
group of systems or all over the world.

Sendmail is the oldest Linux MTA. It was originally derived from the
delivermail program that was used on the ARPANET in 1979. Today there are
several alternative MTAs. In this tutorial, I will also introduce you to
some of these other MTAs.

This tutorial helps you prepare for Objective 108.3 in Topic 108 of the
Linux Server Professional (LPIC-1) exam 102. The objective has a weight of
3.

Prerequisites

To get the most from the tutorials in this series, you need a basic
knowledge of Linux and a working Linux system on which you can practice
the commands covered in this tutorial. You should be familiar with GNU and
UNIX commands. Sometimes different versions of a program format output
differently, so your results might not always look exactly like the
listings shown here.

In this tutorial, I use Ubuntu 14.04 LTS and sendmail 8.14.4 for the
sendmail examples.

Mail transfer

Mail transfer agents such as sendmail deliver mail between users and between systems.
Most Internet mail uses the Simple Mail Transfer Protocol (SMTP), but local mail may
be transferred through files or sockets among other possibilities. Mail is a store
and forward operation, so mail is stored in some kind of file or database until a
user collects it or a receiving system or communication link is available.
Configuring and securing an MTA is quite a complex task, most of which is beyond the
scope of this introductory tutorial.

The mail command

If you use SMTP email, you probably know that there are many, many mail
clients that you can use, including mail, mutt,
alpine, notmuch, and a host of other console and
graphical mail clients. The mail command is an old standby
that can be used to script the sending of mail as well as receive and
manage your incoming mail.

You can use mail interactively to send messages by passing a
list of addressees, or with no arguments you can use it to look at your
incoming mail. Listing 1 shows how to
send a message to user steve and user pat on your system with a carbon
copy to user bob. When prompted for the cc:user and the subject, enter the
body and complete the message by pressing Ctrl+D (hold
down the Ctrl key and press D).

Listing 1. Using mail
interactively to send mail
[email protected]:~$ mail steve,pat
Cc: bob
Subject: Test message 1
This is a test message
 
Ian

If all is well, your mail is sent. If there is an error, you will see an
error message. For example, if you typed an invalid name as a recipient,
the mail is not sent. Note that in this example, all users are on your
local system and therefore all must be valid users.

You can also send mail non-interactively using the command line. Listing 2 shows how to send a small message
to users steve and pat. This capability is particularly useful in scripts.
Different versions of the mail command are available in
different packages. Some support a -c option for cc:, but the
version I am using here does not, so I specify only the to: addresses.

Listing 2. Using mail
non-interactively
[email protected]:~$ mail -t steve,pat  -s "Test message 2" <<< "Another test.nnIan"

If you use mail with no options you will see a list of your
incoming mail as shown in Listing 3. You
see that user steve has the two messages I sent above, plus an earlier one
from me and a later one from user bob. All the mail is marked as ‘N’ for
new mail.

Listing 3. Using mail for
incoming mail
[email protected]:~$ mail
"/var/mail/steve": 4 messages 4 new
>N   1 Ian Shields        Tue Dec 12 21:03  16/704   test message
 N   2 Ian Shields        Tue Dec 12 21:04  18/701   Test message 1
 N   3 Ian Shields        Tue Dec 12 21:23  15/661   Test message 2
 N   4 Bob C              Tue Dec 12 21:45  17/653   How about lunch tomorrow?
?

The currently selected message is shown with a ‘>’, which is message number 1 in
Listing 3. If you press
Enter,
the first page of the next unread message will be displayed. Press the Space
bar
to page through the message. When you finish reading the message
and return to the ‘?’ prompt, press Enter again to view the next
message, and so on. At any ‘?’ prompt you can type ‘h’ to see the list of message
headers again. The ones you have read will now show ‘R’ in the status as shown in
Listing 4.

Listing 4. Using ‘h’ to display mail
headers
? h
 R   1 Ian Shields        Tue Dec 12 21:03  16/704   test message
 R   2 Ian Shields        Tue Dec 12 21:04  18/701   Test message 1
>R   3 Ian Shields        Tue Dec 12 21:23  15/661   Test message 2
 N   4 Bob C              Tue Dec 12 21:45  17/653   How about lunch tomorrow?
?

Here Steve has read the three messages from Ian but has not read the
message from Bob. You can select individual messages by number, and you
can also delete messages that you don’t want by typing ‘d’, or ‘3d’ to
delete the third message. If you type ‘q’ you will quit the
mail command. Messages that you have read will be transferred to the mbox
file in your home directory and the unread messages will remain in your
inbox, by default in /var/mail/$(id -un). See Listing 5.

Listing 5. Using ‘q’ to quit
mail
? h
 R   1 Ian Shields        Tue Dec 12 21:03  16/704   test message
 R   2 Ian Shields        Tue Dec 12 21:04  18/701   Test message 1
>R   3 Ian Shields        Tue Dec 12 21:23  15/661   Test message 2
 N   4 Bob C              Tue Dec 12 21:45  17/653   How about lunch tomorrow?
? q
Saved 3 messages in /home/steve/mbox
Held 1 message in /var/mail/steve
You have mail in /var/mail/steve

If you type ‘x’ to exit instead of ‘q’ to quit, your mailbox will be left
unchanged. Because this is on the /var file system, your system
administrator may allow mail to be kept there only for a limited time. To
reread or otherwise process mail that has been saved to your local mbox
file, use the -f option to specify the file you want to read.
For example mail -f mbox.

Mail aliases

In the previous section you saw how mail can be sent to various users on a
system. You can use a fully qualified name, such as
[email protected] to send mail to a user on another system.

Sometimes you might want all the mail for a user to go to some other place.
For example, you may have a server farm and want all the root mail to go
to a central system administrator. Or you may want to create a mailing
list where mail goes to several people. To do this, you use aliases that
allow you to define one or more destinations for a given user name. The
destinations may be other user mail boxes, files, pipes, or commands that
do further processing. You do this by specifying the aliases in
/etc/mail/aliases or /etc/aliases. Depending on your system, you may find
that one of these is a symbolic link to the other, or you may have only
one of them. You need root authority to change the aliases file.

The general form of an alias is
name: addr_1, addr_2, addr_3, …
where the name is a local user name to alias or an alias and the addr_1,
addr_2, … are one or more aliases. Aliases can be a local user, a local
file name, another alias, a command, an include file, or an external
address.

So how does sendmail distinguish the aliases (the addr-N values)?

  • A local user name is a text string that matches the name of a user on
    this system. Technically this means it can be found using the
    getpwnam call
    .
  • A local file name is a full path and file name that starts with ‘/’.
    It must be writeable by sendmail. Messages are appended
    to the file.
  • A command starts with the pipe symbol (|). Messages are sent to the
    command using standard input.
  • An include file alias starts with :include: and specifies a path and
    file name. The aliases in file are added to the aliases for this name.
  • An external address is an email address such as [email protected]

You should find an example file, such as
/usr/share/sendmail/examples/db/aliases that was installed with your
sendmail package. It contains some recommended aliases for postmaster,
MAILER-DAEMON, abuse, and spam. In Listing 6, I have combined entries from the
example file on my Ubuntu 14.04 LTS system with some rather artificial
examples that illustrate several of the possibilities.

Listing 6. Somewhat artificial
/etc/mail/aliases example
[email protected]:~$ cat /etc/mail/aliases
# First include some default system aliases from
# /usr/share/sendmail/examples/db/aliases

#
# Mail aliases for sendmail
#
# You must run newaliases(1) after making changes to this file.
#

# Required aliases
postmaster:	root
MAILER-DAEMON:	postmaster

# Common aliases
abuse:		postmaster
spam:		postmaster

# Other aliases

# Send steve's mail to bob and pat instead
steve: bob,pat

# Send pat's mail to a file in her home directory and also to her inbox.
# Finally send it to a command that will make another copy.
pat: /home/pat/accumulated-mail,
  pat,
  |/home/pat/makemailcopy.sh

# Mailing list for system administrators
sysadmins: :include: /etc/aliases-sysadmins

Note that pat is both an alias and a user of the system. Alias expansion is
recursive, so if an alias is also a name, then it will be expanded.
Sendmail does not send mail twice to a given user, so if you just put
‘pat’ as an alias for ‘pat’, then it would be ignored since sendmail had
already found and processed ‘pat’. To avoid this problem, you prefix an
alias name with a ” to indicate that it is a name not subject to further
aliasing. This way, pat’s mail can be sent to her normal inbox as well as
the file and command.

Lines in the aliases that start with ‘$’ are comments and are ignored.
Lines that start with blanks are treated as continuation lines.

The include file /etc/aliases-sysadmins is shown in Listing 7.

Listing 7. The /etc/aliases-sysadmins
include file
[email protected]:~$ cat /etc/aliases-sysadmins

# Mailing list for system administrators
bob,pat

The newaliases command

Most configuration files used by sendmail are compiled into database files.
This is also true for mail aliases. You use the newaliases
command to compile your /etc/mail/aliases and any included files to
/etc/mail/aliases.db. Note that newaliases is equivalent to
sendmail -bi. Listing 8 shows an example.

Listing 8. Rebuild the database for the
mail aliases file
[email protected]:~$ sudo newaliases
/etc/mail/aliases: 7 aliases, longest 62 bytes, 184 bytes total
[email protected]:~$ ls -l /etc/mail/aliases*
lrwxrwxrwx 1 root  smmsp    10 Dec  8 15:48 /etc/mail/aliases -> ../aliases
-rw-r----- 1 smmta smmsp 12288 Dec 13 23:18 /etc/mail/aliases.db

Examples of using aliases

Listing 9 shows a simple shell script
that is used as a command in my alias example.

Listing 9. The makemailcopy.sh
script
[email protected]:~$ cat ~pat/makemailcopy.sh
#!/bin/bash

# Note: Target file ~/mail-copy must be writeable by sendmail!
cat >> ~pat/mail-copy

Listing 10 shows the files that are
updated when you put all this to the test.

Listing 10. The /etc/aliases-sysadmins
include file
[email protected]:~$ date
Wed Dec 13 22:54:22 EST 2017
[email protected]:~$ mail -t sysadmins -s "sysadmin test 1" <<< "Testing mail"
[email protected]:~$ ls -lrt $(find /var/mail ~pat -type f -mmin -3 2>/dev/null )
-rw-rw---- 1 pat mail  2046 Dec 13 22:54 /home/pat/mail-copy
-rw------- 1 pat mail 13240 Dec 13 22:54 /var/mail/pat
-rw-rw---- 1 pat mail  9442 Dec 13 22:54 /home/pat/accumulated-mail
-rw-rw---- 1 bob mail 12522 Dec 13 22:54 /var/mail/bob

Some points to note:

  • There is a user ‘mail’ with group name ‘mail’ that is used by
    sendmail.
  • User mail is stored by sendmail in /var/mail which is also the home
    directory of user ‘mail’. The inbox for user ‘ian’ defaults to
    /var/mail/ian.
  • If you want sendmail to write files in a user directory, the file must
    be writeable by sendmail. Rather than making it world writeable, it is
    customary to make it group writeable and make the group ‘mail’. You
    may need a system administrator to do this for you.

Using a .forward file to forward mail

The aliases file must be managed by a system administrator. Individual
users can enable forwarding of their own mail using a .forward file in
their own home directory. You can put anything in your .forward file that
is allowed on the right side of the aliases file. The file contains plain
text and does not need to be compiled. When mail is destined for you,
sendmail checks for a .forward file in your home directory and processes
the entries the same way it processes aliases.

Mail queues and the mailq command

Linux mail handling uses a store-and-forward model. You have already seen
that your incoming mail is stored in a file in /var/mail until you read
it. Outgoing mail is also stored until a receiving server connection is
available. You use the mailq command to see what mail is
queued. Listing 11 shows an example of
mail being sent to an external user, [email protected], and the result of
running the mailq command. In this case, there is currently
no active link to attic4-c6, so the mail will remain queued until a link
becomes active.

Listing 11. Using the mailq
command
[email protected]:~$ mail -t [email protected] -s "External mail" <<< "Testing external mail queues"
[email protected]:~$ mailq
MSP Queue status...
/var/spool/mqueue-client is empty
		Total requests: 0
MTA Queue status...
		/var/spool/mqueue (1 request)
-----Q-ID----- --Size-- -----Q-Time----- ------------Sender/Recipient-----------
vBE4mdE7025908*      29 Wed Dec 13 23:48 <[email protected]>
					 <[email protected]>
		Total requests: 1

Other mail transfer agents

In response to security issues with sendmail, several other mail transfer
agents were developed during the 1990’s. Postfix is perhaps the most
popular, but qmail and exim are also widely used.

Postfix started life at IBM research as an alternative to sendmail. It
attempts to be fast, easy to administer, and secure. The outside looks
somewhat like sendmail, but the inside is completely different.

Qmail is a secure, reliable, efficient, simple message transfer agent
developerd by Dan Bernstein. However, the core qmail package has not been
updated for many years. Qmail and several other packages have now been
collected into IndiMail.

Exim is another MTA developed at the University of Cambridge. Originally,
the name stood for EXperimental Internet Mailer.

All of these MTAs were designed as sendmail replacements, so they all have some form
of sendmail compatibility. Each can handle aliases and .forward files. Some provide
a sendmail command as a front end to the particular MTA’s own command.
Most allow the usual sendmail options, although some options might be ignore
silently.
The
mailq command is supported directly or by an alternate command with
a similar function. For example, you can use mailq or
exim -bp to display the exim mail queue. Needless to say,
output can look different compared to that produced by sendmail’s mailq
command.

See Related topics where you can find more
information on all of these MTAs.

This concludes your introduction to mail transfer agents on Linux.


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