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Mail - postfix

Postfix is the default Mail Transfer Agent (MTA) in Ubuntu. It attempts to be fast and secure, with flexibility in administration. It is compatible with the MTA sendmail. This section will explain installation, including how to configure SMTP for secure communications.

Note

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

Installation

To install Postfix run the following command:

sudo apt install postfix

For now, it is ok to simply accept defaults by pressing return for each question. Some of the configuration options will be investigated in greater detail in the next stage.

Deprecation warning: please note that the mail-stack-delivery metapackage has been deprecated in Focal. The package still exists for compatibility reasons, but won’t setup a working email system.

Basic Configuration

There are four things you should decide before starting configuration:

  • The <Domain> for which you’ll accept email (we’ll use mail.example.com in our example)
  • The network and class range of your mail server (we’ll use 192.168.0.0/24)
  • The username (we’re using steve)
  • Type of mailbox format (mbox is default, we’ll use the alternative, Maildir)

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

To set the mailbox format, you can either edit the configuration file directly, or use the postconf command. In either case, 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 the SASL authentication mechanism, using Transport Layer Security (TLS) to encrypt the authentication process. Once authenticated the SMTP server will allow the client to relay mail.

To configure Postfix for SMTP-AUTH using SASL (Dovecot SASL), run these commands at a terminal prompt:

sudo postconf -e 'smtpd_sasl_type = dovecot'
sudo postconf -e 'smtpd_sasl_path = private/auth'
sudo postconf -e 'smtpd_sasl_local_domain ='
sudo postconf -e 'smtpd_sasl_security_options = noanonymous,noplaintext'
sudo postconf -e 'smtpd_sasl_tls_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'

Note

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

There are several SASL mechanism properties worth evaluating to improve the security of your deployment. The options “noanonymous,noplaintext” prevent use of mechanisms that permit anonymous authentication or that transmit credentials unencrypted.

Next, generate or obtain a digital certificate for TLS. See security - certificates in this guide for details about generating digital certificates and setting up your own Certificate Authority (CA).

Note

MUAs connecting to your mail server via TLS will need to recognize the certificate used for TLS. This can either be done using a certificate from Let’s Encrypt, from a commercial CA or with a self-signed certificate that users manually install/accept. For MTA to MTA TLS certficates are never validated without advance agreement from the affected organizations. For MTA to MTA TLS, unless local policy requires it, there is no reason not to use a self-signed certificate. Refer to security - certificates in this guide for more details.

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

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 'myhostname = mail.example.com'

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 security - certificates in this guide.

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:

# See /usr/share/postfix/main.cf.dist for a commented, more complete
# version

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

myhostname = server1.example.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = server1.example.com, localhost.example.com, localhost
relayhost =
mynetworks = 127.0.0.0/8
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
smtpd_sasl_local_domain =
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions =
permit_sasl_authenticated,permit_mynetworks,reject _unauth_destination
smtpd_tls_auth_only = no
smtp_tls_security_level = may
smtpd_tls_security_level = may
smtp_tls_note_starttls_offer = yes
smtpd_tls_key_file = /etc/ssl/private/smtpd.key
smtpd_tls_cert_file = /etc/ssl/certs/smtpd.crt
smtpd_tls_CAfile = /etc/ssl/certs/cacert.pem
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom

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

sudo systemctl restart postfix.service

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.

When using ipv6, the mynetworks parameter may need to be modified to allow ipv6 addresses, for example:

 mynetworks = 127.0.0.0/8, [::1]/128

Configuring SASL

Postfix supports two SASL implementations: Cyrus SASL and Dovecot SASL. To enable Dovecot SASL the dovecot-core package will need to be installed:

sudo apt install dovecot-core

Next, edit /etc/dovecot/conf.d/10-master.conf and change the following:

service auth {
  # auth_socket_path points to this userdb socket by default. It's typically
  # used by dovecot-lda, doveadm, possibly imap process, etc. Its default
  # permissions make it readable only by root, but you may need to relax these
  # permissions. Users that have access to this socket are able to get a list
  # of all usernames and get results of everyone's userdb lookups.
  unix_listener auth-userdb {
    #mode = 0600
    #user = 
    #group = 
  }

  # Postfix smtp-auth
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
  }
 }

To permit use of SMTP-AUTH by Outlook clients, change the following line in the authentication mechanisms section of /etc/dovecot/conf.d/10-auth.conf from:

auth_mechanisms = plain

to this:

auth_mechanisms = plain login

Once you have Dovecot configured, restart it with:

sudo systemctl restart dovecot.service

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 in the output, then everything is working perfectly. Type quit to exit.

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

Troubleshooting

When problems arise, there are a few common ways to diagnose the cause.

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 usage, locate 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 service postfix restart

SMTPS

If you need secure SMTP, edit /etc/postfix/master.cf and uncomment the following line:

smtps     inet  n       -       -       -       -       smtpd
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
  -o milter_macro_daemon_name=ORIGINATING

Log Viewing

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

Increasing Logging Detail

The amount of detail that is recorded in the logs can be increased via the configuration options. For example, 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'

Reload the service after any configuration change, to make the new config active:

    sudo systemctl reload postfix.service

Logging mail delivery

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'
    sudo systemctl reload postfix.service

Increasing daemon verbosity

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

Then, reload the service as usual:

    sudo systemctl reload postfix.service

Logging SASL debug info

To increase the amount of information logged when troubleshooting SASL issues you can set the following options in /etc/dovecot/conf.d/10-logging.conf

    auth_debug=yes
    auth_debug_passwords=yes

Just like Postfix if you change a Dovecot configuration the process will need to be reloaded:

    sudo systemctl reload dovecot.service

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.

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