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).

[Note]

This guide does not cover setting up Postfix Virtual Domains, for information on Virtual Domains and other advanced configurations see the section called “References”.

Installation

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]/128 192.168.0.0/24

  • 0

  • +

  • all

[Note]

Replace mail.example.com with the domain for which you'll accept email, 192.168.0.0/24 with the actual network and class range of your mail server, and steve with the appropriate username.

Now is a good time to decide which mailbox format you want to use. By default Postfix will use mbox for the mailbox format. Rather than editing the configuration file directly, you can use the postconf command to configure all postfix parameters. The configuration parameters will be stored in /etc/postfix/main.cf file. Later if you wish to re-configure a particular parameter, you can either run the command or change it manually in the file.

To configure the mailbox format for Maildir:

sudo postconf -e 'home_mailbox = Maildir/'
[Note]

This will place new mail in /home/username/Maildir so you will need to configure your Mail Delivery Agent (MDA) to use the same path.

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.

  1. Configure Postfix for SMTP-AUTH using SASL (Dovecot SASL):

    sudo postconf -e 'smtpd_sasl_type = dovecot'
    sudo postconf -e 'smtpd_sasl_path = private/auth-client'
    sudo postconf -e 'smtpd_sasl_local_domain ='
    sudo postconf -e 'smtpd_sasl_security_options = noanonymous'
    sudo postconf -e 'broken_sasl_auth_clients = yes'
    sudo postconf -e 'smtpd_sasl_auth_enable = yes'
    sudo postconf -e 'smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination'
    sudo postconf -e 'inet_interfaces = all'
    
    [Note]

    The smtpd_sasl_path configuration is a path relative to the Postfix queue directory.

  2. Next, obtain a digital certificate for TLS. See the section called “Certificates” for details. This example also uses a Certificate Authority (CA). For information on generating a CA certificate see the section called “Certification Authority”.

    [Note]

    You can get the digital certificate from a certificate authority. But unlike web clients, SMTP clients rarely complain about "self-signed certificates", so alternatively, you can create the certificate yourself. Refer to the section called “Creating a Self-Signed Certificate” for more details.

  3. Once you have a certificate, configure Postfix to provide TLS encryption for both incoming and outgoing mail:

    sudo postconf -e 'smtpd_tls_auth_only = no'
    sudo postconf -e 'smtp_tls_security_level = may'
    sudo postconf -e 'smtpd_tls_security_level = may'
    sudo postconf -e 'smtp_tls_note_starttls_offer = yes'
    sudo postconf -e 'smtpd_tls_key_file = /etc/ssl/private/server.key'
    sudo postconf -e 'smtpd_tls_cert_file = /etc/ssl/certs/server.crt'
    sudo postconf -e 'smtpd_tls_loglevel = 1'
    sudo postconf -e 'smtpd_tls_received_header = yes'
    sudo postconf -e 'smtpd_tls_session_cache_timeout = 3600s'
    sudo postconf -e 'tls_random_source = dev:/dev/urandom'
    sudo postconf -e 'myhostname = mail.example.com'
    
  4. If you are using your own Certificate Authority to sign the certificate enter:

    sudo postconf -e 'smtpd_tls_CAfile = /etc/ssl/certs/cacert.pem'
    

    Again, for more details about certificates see the section called “Certificates”.

[Note]

After running all the commands, Postfix is configured for SMTP-AUTH and a self-signed certificate has been created for TLS encryption.

Now, the file /etc/postfix/main.cf should look like this.

The postfix initial configuration is complete. Run the following command to restart the postfix daemon:

sudo /etc/init.d/postfix restart

Postfix supports SMTP-AUTH as defined in RFC2554. It is based on SASL. However it is still necessary to set up SASL authentication before you can use SMTP-AUTH.

Configuring SASL

Postfix supports two SASL implementations Cyrus SASL and Dovecot SASL. To enable Dovecot SASL the dovecot-common package will need to be installed. From a terminal prompt enter the following:

sudo apt-get install dovecot-common

Next you will need to edit /etc/dovecot/dovecot.conf. In the auth default section uncomment the socket listen option and change the following:

  socket listen {
    #master {
      # Master socket provides access to userdb information. It's typically
      # used to give Dovecot's local delivery agent access to userdb so it
      # can find mailbox locations.
      #path = /var/run/dovecot/auth-master
      #mode = 0600
      # Default user/group is the one who started dovecot-auth (root)
      #user = 
      #group = 
    #}
    client {
      # The client socket is generally safe to export to everyone. Typical use
      # is to export it to your SMTP server so it can do SMTP AUTH lookups
      # using it.
      path = /var/spool/postfix/private/auth-client
      mode = 0660
      user = postfix
      group = postfix
    }
  }

In order to let Outlook clients use SMTPAUTH, in the auth default section of /etc/dovecot/dovecot.conf add "login":

  mechanisms = plain login

Once you have Dovecot configured restart it with:

sudo /etc/init.d/dovecot restart

Postfix-Dovecot

Another option for configuring Postfix for SMTP-AUTH is 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.

[Note]

You may or may not want to run IMAP, IMAPS, POP3, or POP3S on your mail server. For example, if you are configuring your server to be a mail gateway, spam/virus filter, etc. If this is the case it may be easier to use the above commands to configure Postfix for SMTPAUTH.

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 the section called “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
    
[Note]

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
    
[Note]

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

[Note]

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.

Also, the Ubuntu Wiki Postifx page has more information.