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Chapter 8. Deploying and configuring a Postfix SMTP server

As a system administrator, you can configure your email infrastructure by using a mail transport agent (MTA), such as Postfix, to transport email messages between hosts using the SMTP protocol. Postfix is a server-side application for routing and delivering mail. You can use Postfix to set up a local mail server, create a null-client mail relay, use a Postfix server as a destination for multiple domains, or choose an LDAP directory instead of files for lookups.

The key features of Postfix:

  • Security features to protect against common email related threats
  • Customization options, including support for virtual domains and aliases

8.1. Overview of the main Postfix configuration files

The postfix package provides multiple configuration files in the /etc/postfix/ directory.

To configure your email infrastructure, use the following configuration files:

  • main.cf — contains the global configuration of Postfix.
  • master.cf — specifies Postfix interaction with various processes to accomplish mail delivery.
  • access — specifies access rules, for example hosts that are allowed to connect to Postfix.
  • transport — maps email addresses to relay hosts.
  • aliases — contains a configurable list required by the mail protocol that describes user ID aliases. Note that you can find this file in the /etc/ directory.

8.2. Installing and configuring a Postfix SMTP server

You can configure your Postfix SMTP server to receive, store, and deliver email messages. If the mail server package is not selected during the system installation, Postfix will not be available by default. Perform the following steps to install Postfix:

Prerequisites

Procedure

  1. Install Postfix: 

    # yum install postfix

  2. To configure Postfix, edit the /etc/postfix/main.cf file and make the following changes:

    1. By default, Postfix receives emails only on the loopback interface. To configure Postfix to listen on specific interfaces, update the inet_interfaces parameter to the IP addresses of these interfaces: 

      inet_interfaces = 127.0.0.1/32, [::1]/128, 192.0.2.1, [2001:db8:1::1]

      To configure Postfix to listen on all interfaces, set: 

      inet_interfaces = all

    2. If you want that Postfix uses a different hostname than the fully-qualified domain name (FQDN) that is returned by the gethostname() function, add the myhostname parameter: 

      myhostname = <smtp.example.com>

      For example, Postfix adds this hostname to header of emails it processes.

    3. If the domain name differs from the one in the myhostname parameter, add the mydomain parameter: 

      mydomain = <example.com>

    4. Add the myorigin parameter and set it to the value of mydomain: 

      myorigin = $mydomain

      With this setting, Postfix uses the domain name as origin for locally posted mails instead of the hostname.

    5. Add the mynetworks parameter, and define the IP ranges of trusted networks that are allowed to send mails: 

      mynetworks = 127.0.0.1/32, [::1]/128, 192.0.2.1/24, [2001:db8:1::1]/64

      If clients from not trustworthy networks, such as the Internet, should be able to send mails through this server, you must configure relay restrictions in a later step.

  3. Verify if the Postfix configuration in the main.cf file is correct: 

    $ postfix check

  4. Enable the postfix service to start at boot and start it: 

    # systemctl enable --now postfix

  5. Allow the smtp traffic through firewall and reload the firewall rules: 

    # firewall-cmd --permanent --add-service smtp

# firewall-cmd --reload

Verification

  1. Verify that the postfix service is running: 

    # systemctl status postfix

    • Optional: Restart the postfix service, if the output is stopped, waiting, or the service is not running: 

      # systemctl restart postfix

    • Optional: Reload the postfix service after changing any options in the configuration files in the /etc/postfix/ directory to apply those changes: 

      # systemctl reload postfix

  2. Verify the email communication between local users on your system: 

    # echo "This is a test message" | mail -s <SUBJECT> <user@mydomain.com>

  3. Verify that your mail server is not an open relay by sending an email to an email address outside of your domain from a client (server1) to your mail server (server2):

    1. Edit the /etc/postfix/main.cf file for server1 as follows: 

      relayhost = <ip_address_of_server2>

    2. Edit the /etc/postfix/main.cf file for server2 as follows: 

      mynetworks = <ip_address_of_server2>

    3. On server1, send the following mail: 

      # echo "This is an open relay test message" | mail -s <SUBJECT> <user@example.com>

    4. Check the /var/log/maillog file: 

      554 Relay access denied - the server is not going to relay.
250 OK or similar - the server is going to relay.

Troubleshooting

  • In case of errors, check the /var/log/maillog file.

Additional resources

8.3. Customizing TLS settings of a Postfix server

To make your email traffic encrypted and therefore more secure, you can configure Postfix to use a certificate from a trusted certificate authority (CA) instead of the self-signed certificate and customize the Transport Layer Security (TLS) security settings. In RHEL 8, the TLS encryption protocol is enabled in the Postfix server by default. The basic Postfix TLS configuration contains self-signed certificates for inbound SMTP and the opportunistic TLS for outbound SMTP.

Prerequisites

  • You have the root access.
  • You have the postfix package installed on your server.
  • You have a certificate signed by a trusted certificate authority (CA) and a private key.

  • You have copied the following files to the Postfix server:

    • The server certificate: /etc/pki/tls/certs/postfix.pem
    • The private key: /etc/pki/tls/private/postfix.key

Procedure

  1. Set the path to the certificate and private key files on the server where Postfix is running by adding the following lines to the /etc/postfix/main.cf file: 

    smtpd_tls_cert_file = /etc/pki/tls/certs/postfix.pem
smtpd_tls_key_file = /etc/pki/tls/private/postfix.key

  2. Restrict the incoming SMTP connections to authenticated users only by editing the /etc/postfix/main.cf file: 

    smtpd_tls_auth_only = yes

  3. Reload the postfix service to apply the changes: 

    # systemctl reload postfix

Verification

  • Configure your client to use TLS encryption and send an email.

    Note

    To get additional information about Postfix client TLS activity, increase the log level from 0 to 1 by changing the following line in the /etc/postfix/main.cf: 

    smtp_tls_loglevel = 1

8.4. Configuring Postfix to forward all emails to a mail relay

If you want to forward all email to a mail relay, you can configure Postfix server as a null client. In this configuration Postfix only forwards mail to a different mail server and is not capable of receiving mail.

Prerequisites

  • You have the root access.
  • You have the postfix package installed on your server.
  • You have the IP address or hostname of the relay host to which you want to forward emails.

Procedure

  1. To prevent Postfix from accepting any local email delivery and making it a null client, edit the /etc/postfix/main.cf file and make the following changes:

    1. Configure Postfix to forward all email by setting the mydestination parameter equal to an empty value: 

      mydestination =

      In this configuration the Postfix server is not a destination for any email and acts as a null client.

    2. Specify the mail relay server that receives the email from your null client: 

      relayhost = <[ip_address_or_hostname]>

      The relay host is responsible for the mail delivery. Enclose <ip_address_or_hostname> in square brackets.

    3. Configure the Postfix mail server to listen only on the loopback interface for emails to deliver: 

      inet_interfaces = loopback-only

    4. If you want Postfix to rewrite the sender domain of all outgoing emails to the company domain of your relay mail server, set: 

      myorigin = <relay.example.com>

    5. To disable the local mail delivery, add the following directive at the end of the configuration file: 

      local_transport = error: local delivery disabled

    6. Add the mynetworks parameter so that Postfix forwards email from the local system originating from the 127.0.0.0/8 IPv4 network and the [::1]/128 IPv6 network to the mail relay server: 

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

  2. Verify if the Postfix configuration in the main.cf file is correct: 

    $ postfix check

  3. Restart the postfix service to apply the changes: 

    # systemctl restart postfix

Verification

  • Verify that the email communication is forwarded to the mail relay: 

    # echo "This is a test message" | mail -s <SUBJECT> <user@example.com>

Troubleshooting

  • In case of errors, check the /var/log/maillog file.

Additional resources

  • The /etc/postfix/main.cf configuration file

8.5. Configuring Postfix as a destination for multiple domains

You can configure Postfix as a mail server that can receive emails for multiple domains. In this configuration, Postfix acts as the final destination for emails sent to addresses within the specified domains. You can configure the following:

  • Set up multiple email addresses that point to the same email destination
  • Route incoming email for multiple domains to the same Postfix server

Prerequisites

  • You have the root access.
  • You have configured a Postfix server.

Procedure

  1. In the /etc/postfix/virtual virtual alias file, specify the email addresses for each domain. Add each email address on a new line: 

    <info@example.com>  <user22@example.net>
<sales@example.com>  <user11@example.org>

    In this example, Postfix redirects all emails sent to info@example.com to user22@example.net and email sent to sales@example.com to user11@example.org.

  2. Create a hash file for the virtual alias map: 

    # postmap /etc/postfix/virtual

    This command creates the /etc/postfix/virtual.db file. Note that you must always re-run this command after you update the /etc/postfix/virtual file.

  3. In the Postfix /etc/postfix/main.cf configuration file, add the virtual_alias_maps parameter and point it to the hash file: 

    virtual_alias_maps = hash:/etc/postfix/virtual

  4. Reload the postfix service to apply the changes: 

    # systemctl reload postfix

Verification

  • Test the configuration by sending an email to one of the virtual email addresses.

Troubleshooting

  • In case of errors, check the /var/log/maillog file.

8.6. Using an LDAP directory as a lookup table

If you use a Lightweight Directory Access Protocol (LDAP) server to store accounts, domains or aliases, you can configure Postfix to use the LDAP server as a lookup table. Using LDAP instead of files for lookups enables you to have a central database.

Prerequisites

  • You have the root access.
  • You have the postfix package installed on your server.
  • You have an LDAP server with the required schema and user credentials.
  • You have the postfix-ldap plugin installed on the server running Postfix.

Procedure

  1. Configure the LDAP lookup parameters by creating a /etc/postfix/ldap-aliases.cf file with the following content:

    1. Specify the hostname of the LDAP server: 

      server_host = <ldap.example.com>

    2. Specify the base domain name for the LDAP search: 

      search_base = dc=<example>,dc=<com>
    3. Optional: Customize the LDAP search filter and attributes based on your requirements. The filter for searching the directory defaults to query_filter = mailacceptinggeneralid=%s.

  2. Enable the LDAP source as a lookup table in the /etc/postfix/main.cf configuration file by adding the following content: 

    virtual_alias_maps = ldap:/etc/postfix/ldap-aliases.cf

  3. Verify the LDAP configuration by running the postmap command, which checks for any syntax errors or connectivity issues: 

    # postmap -q @<example.com> ldap:/etc/postfix/ldap-aliases.cf

  4. Reload the postfix service to apply the changes: 

    # systemctl reload postfix

Verification

  • Send a test email to verify that the LDAP lookup works correctly. Check the mail logs in /var/log/maillog for any errors.

Additional resources

  • /usr/share/doc/postfix/README_FILES/LDAP_README file
  • /usr/share/doc/postfix/README_FILES/DATABASE_README file

8.7. Configuring Postfix as an outgoing mail server to relay for authenticated users

You can configure Postfix to relay mail for authenticated users. In this scenario, you allow users to authenticate themselves and use their email address to send mail through your SMTP server by configuring Postfix as an outgoing mail server with SMTP authentication, TLS encryption, and sender address restrictions.

Prerequisites

  • You have the root access.
  • You have configured a Postfix server.

Procedure

  1. To configure Postfix as an outgoing mail server, edit the /etc/postfix/main.cf file and add the following:

    1. Enable SMTP authentication: 

      smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes

    2. Disable access without TLS: 

      smtpd_tls_auth_only = yes

    3. Allow mail relaying only for authenticated users: 

      smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination

    4. Optional: Restrict users to use their own email address only as a sender: 

      smtpd_sender_restrictions = reject_sender_login_mismatch

  2. Reload the postfix service to apply the changes: 

    # systemctl reload postfix

Verification

  • Authenticate in your SMTP client that supports TLS and SASL. Send an test email to verify that the SMTP authentication works correctly.

8.8. Delivering email from Postfix to Dovecot running on the same host

You can configure Postfix to deliver incoming mail to Dovecot on the same host using LMTP over a UNIX socket. This socket enables direct communication between Postfix and Dovecot on the local machine.

Prerequisites

Procedure

  1. Configure Postfix to use the LMTP protocol and the UNIX domain socket for delivering mail to Dovecot in the /etc/postfix/main.cf file:

    • If you want to use virtual mailboxes, add the following content: 

      virtual_transport = lmtp:unix:/var/run/dovecot/lmtp

    • If you want to use non-virtual mailboxes, add the following content: 

      mailbox_transport = lmtp:unix:/var/run/dovecot/lmtp

  2. Reload postfix to apply the changes: 

    # systemctl reload postfix

Verification

  • Send an test email to verify that the LMTP socket works correctly. Check the mail logs in /var/log/maillog for any errors.

8.9. Delivering email from Postfix to Dovecot running on a different host

You can establish a secure connection between Postfix mail server and the Dovecot delivery agent over the network. To do so, configure the LMTP service to use network socket for delivering mail between mail servers. By default, the LMTP protocol is not encrypted. However, if you configured TLS encryption, Dovecot uses the same settings automatically for the LMTP service. SMTP servers can then connect to it using the STARTTLS command over LMTP.

Prerequisites

Procedure

  1. Configure Postfix to use the LMTP protocol and the INET domain socket for delivering mail to Dovecot in the /etc/postfix/main.cf file by adding the following content: 

    mailbox_transport = lmtp:inet:<dovecot_host>:<port>

    Replace <dovecot_host> with the IP address or hostname of the Dovecot server and <port> with the port number of the LMTP service.

  2. Reload the postfix service to apply the changes: 

    # systemctl reload postfix

Verification

  • Send an test email to an address hosted by the remote Dovecot server and check the Dovecot logs to ensure that the mail was successfully delivered.

8.10. Securing the Postfix service

Postfix is a mail transfer agent (MTA) that uses the Simple Mail Transfer Protocol (SMTP) to deliver electronic messages between other MTAs and to email clients or delivery agents. Although MTAs can encrypt traffic between one another, they might not do so by default. You can also mitigate risks to various attacks by changing setting to more secure values.

8.10.2. Postfix configuration options for limiting DoS attacks

An attacker can flood the server with traffic, or send information that triggers a crash, causing a denial of service (DoS) attack. You can configure your system to reduce the risk of such attacks by setting limits in the /etc/postfix/main.cf file. You can change the value of the existing directives or you can add new directives with custom values in the <directive> = <value> format.

Use the following list of directives for limiting a DoS attack:

smtpd_client_connection_rate_limit
This directive limits the maximum number of connection attempts any client can make to this service per time unit. The default value is 0, which means a client can make as many connections per time unit as Postfix can accept. By default, the directive excludes clients in trusted networks.
anvil_rate_time_unit
This directive is a time unit to calculate the rate limit. The default value is 60 seconds.
smtpd_client_event_limit_exceptions
This directive excludes clients from the connection and rate limit commands. By default, the directive excludes clients in trusted networks.
smtpd_client_message_rate_limit
This directive defines the maximum number of message deliveries from client to request per time unit (regardless of whether or not Postfix actually accepts those messages).
default_process_limit
This directive defines the default maximum number of Postfix child processes that provide a given service. You can ignore this rule for specific services in the master.cf file. By default, the value is 100.
queue_minfree
This directive defines the minimum amount of free space required to receive mail in the queue file system. The directive is currently used by the Postfix SMTP server to decide if it accepts any mail at all. By default, the Postfix SMTP server rejects MAIL FROM commands when the amount of free space is less than 1.5 times the message_size_limit. To specify a higher minimum free space limit, specify a queue_minfree value that is at least 1.5 times the message_size_limit. By default, the queue_minfree value is 0.
header_size_limit
This directive defines the maximum amount of memory in bytes for storing a message header. If a header is large, it discards the excess header. By default, the value is 102400 bytes.
message_size_limit
This directive defines the maximum size of a message including the envelope information in bytes. By default, the value is 10240000 bytes.

8.10.3. Configuring Postfix to use SASL

Postfix supports Simple Authentication and Security Layer (SASL) based SMTP Authentication (AUTH). SMTP AUTH is an extension of the Simple Mail Transfer Protocol. Currently, the Postfix SMTP server supports the SASL implementations in the following ways:

Dovecot SASL
The Postfix SMTP server can communicate with the Dovecot SASL implementation using either a UNIX-domain socket or a TCP socket. Use this method if Postfix and Dovecot applications are running on separate machines.
Cyrus SASL
When enabled, SMTP clients must authenticate with the SMTP server using an authentication method supported and accepted by both the server and the client.

Prerequisites

  • The dovecot package is installed on the system

Procedure

  1. Set up Dovecot:

    1. Include the following lines in the /etc/dovecot/conf.d/10-master.conf file: 

      service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
  }
}

      The previous example uses UNIX-domain sockets for communication between Postfix and Dovecot. The example also assumes default Postfix SMTP server settings, which include the mail queue located in the /var/spool/postfix/ directory, and the application running under the postfix user and group.

    2. Optional: Set up Dovecot to listen for Postfix authentication requests through TCP: 

      service auth {
  inet_listener {
      port = port-number
  }
}

    3. Specify the method that the email client uses to authenticate with Dovecot by editing the auth_mechanisms parameter in /etc/dovecot/conf.d/10-auth.conf file: 

      auth_mechanisms = plain login

      The auth_mechanisms parameter supports different plaintext and non-plaintext authentication methods.

  2. Set up Postfix by modifying the /etc/postfix/main.cf file:

    1. Enable SMTP Authentication on the Postfix SMTP server: 

      smtpd_sasl_auth_enable = yes

    2. Enable the use of Dovecot SASL implementation for SMTP Authentication: 

      smtpd_sasl_type = dovecot

    3. Provide the authentication path relative to the Postfix queue directory. Note that the use of a relative path ensures that the configuration works regardless of whether the Postfix server runs in chroot or not: 

      smtpd_sasl_path = private/auth

      This step uses UNIX-domain sockets for communication between Postfix and Dovecot.

      To configure Postfix to look for Dovecot on a different machine in case you use TCP sockets for communication, use configuration values similar to the following: 

      smtpd_sasl_path = inet: ip-address : port-number

      In the previous example, replace the ip-address with the IP address of the Dovecot machine and port-number with the port number specified in Dovecot’s /etc/dovecot/conf.d/10-master.conf file.

    4. Specify SASL mechanisms that the Postfix SMTP server makes available to clients. Note that you can specify different mechanisms for encrypted and unencrypted sessions. 

      smtpd_sasl_security_options = noanonymous, noplaintext
smtpd_sasl_tls_security_options = noanonymous

      The previous directives specify that during unencrypted sessions, no anonymous authentication is allowed and no mechanisms that transmit unencrypted user names or passwords are allowed. For encrypted sessions that use TLS, only non-anonymous authentication mechanisms are allowed.

Additional resources