Postfix

Postfix is the default Mail Transfer Agent (MTA) in Ubuntu. It attempts to be fast and easy to administer and secure. It is compatible with the MTA sendmail. This section explains how to install and configure postfix. It also explains how to set it up as an SMTP server using a secure connection (for sending emails securely).

Instal·lació

To install postfix run the following command:

sudo apt-get install postfix

Simply press return when the installation process asks questions, the configuration will be done in greater detail in the next stage.

Basic Configuration

To configure postfix, run the following command:

sudo dpkg-reconfigure postfix

The user interface will be displayed. On each screen, select the following values:

  • Internet Site

  • mail.example.com

  • steve

  • mail.example.com, localhost.localdomain, localhost

  • No

  • 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 192.168.0/24

  • 0

  • +

  • all

[Nota]

Replace mail.example.com with your mail server hostname, 192.168.0/24 with the actual network and class range of your mail server, and steve with the appropriate username.

SMTP Authentication

SMTP-AUTH allows a client to identify itself through an authentication mechanism (SASL). Transport Layer Security (TLS) should be used to encrypt the authentication process. Once authenticated the SMTP server will allow the client to relay mail.

Configuring Postfix for SMTP-AUTH is very simple using the dovecot-postfix package. This package will install Dovecot and configure Postfix to use it for both SASL authentication and as a Mail Delivery Agent (MDA). The package also configures Dovecot for IMAP, IMAPS, POP3, and POP3S.

To install the package, from a terminal prompt enter:

sudo apt-get install dovecot-postfix

You should now have a working mail server, but there are a few options that you may wish to further customize. For example, the package uses the certificate and key from the ssl-cert package, and in a production environment you should use a certificate and key generated for the host. See “Certificates” for more details.

Once you have a customized certificate and key for the host, change the following options in /etc/postfix/main.cf:

smtpd_tls_cert_file = /etc/ssl/certs/ssl-mail.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-mail.key

Then restart Postfix:

sudo /etc/init.d/postfix restart

Testing

SMTP-AUTH configuration is complete. Now it is time to test the setup.

To see if SMTP-AUTH and TLS work properly, run the following command:

telnet mail.example.com 25

After you have established the connection to the postfix mail server, type:

ehlo mail.example.com

If you see the following lines among others, then everything is working perfectly. Type quit to exit.

250-STARTTLS
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
250 8BITMIME

Troubleshooting

This section introduces some common ways to determine the cause if problems arise.

Escaping chroot

The Ubuntu postfix package will by default install into a chroot environment for security reasons. This can add greater complexity when troubleshooting problems.

To turn off the chroot operation locate for the following line in the /etc/postfix/master.cf configuration file:

smtp      inet  n       -       -       -       -       smtpd

and modify it as follows:

smtp      inet  n       -       n       -       -       smtpd

You will then need to restart Postfix to use the new configuration. From a terminal prompt enter:

sudo /etc/init.d/postfix restart

Log Files

Postfix sends all log messages to /var/log/mail.log. However error and warning messages can sometimes get lost in the normal log output so they are also logged to /var/log/mail.err and /var/log/mail.warn respectively.

To see messages entered into the logs in real time you can use the tail -f command:

tail -f /var/log/mail.err

The amount of detail that is recorded in the logs can be increased. Below are some configuration options for increasing the log level for some of the areas covered above.

  • To increase TLS activity logging set the smtpd_tls_loglevel option to a value from 1 to 4.

    sudo postconf -e 'smtpd_tls_loglevel = 4'
    
  • If you are having trouble sending or receiving mail from a specific domain you can add the domain to the debug_peer_list parameter.

    sudo postconf -e 'debug_peer_list = problem.domain'
    
  • You can increase the verbosity of any Postfix daemon process by editing the /etc/postfix/master.cf and adding a -v after the entry. For example edit the smtp entry:

    smtp      unix  -       -       -       -       -       smtp -v
    
[Nota]

It is important to note that after making one of the logging changes above the Postfix process will need to be reloaded in order to recognize the new configuration: sudo /etc/init.d/postfix reload

  • To increase the amount of information logged when troubleshooting SASL issues you can set the following options in /etc/dovecot/dovecot.conf

    auth_debug=yes
    auth_debug_passwords=yes
    
[Nota]

Just like Postfix if you change a Dovecot configuration the process will need to be reloaded: sudo /etc/init.d/dovecot reload.

[Nota]

Some of the options above can drastically increase the amount of information sent to the log files. Remember to return the log level back to normal after you have corrected the problem. Then reload the appropriate daemon for the new configuration to take affect.

References

Administering a Postfix server can be a very complicated task. At some point you may need to turn to the Ubuntu community for more experienced help.

A great place to ask for Postfix assistance, and get involved with the Ubuntu Server community, is the #ubuntu-server IRC channel on freenode. You can also post a message to one of the Web Forums.

For in depth Postfix information Ubuntu developers highly recommend: The Book of Postfix.

Finally, the Postfix website also has great documentation on all the different configuration options available.