Chapter 2. 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 2.1. 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. 2.2. 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 You have the root access. Register your system Procedure Disable and remove the Sendmail utility: Install Postfix: To configure Postfix, edit the /etc/postfix/main.cf file and make the following changes: 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: To configure Postfix to listen on all interfaces, set: 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: For example, Postfix adds this hostname to header of emails it processes. If the domain name differs from the one in the myhostname parameter, add the mydomain parameter: Add the myorigin parameter and set it to the value of mydomain : With this setting, Postfix uses the domain name as origin for locally posted mails instead of the hostname. Add the mynetworks parameter, and define the IP ranges of trusted networks that are allowed to send mails: 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. Verify if the Postfix configuration in the main.cf file is correct: Enable the postfix service to start at boot and start it: Allow the smtp traffic through firewall and reload the firewall rules: Verification Verify that the postfix service is running: Optional: Restart the postfix service, if the output is stopped, waiting, or the service is not running: Optional: Reload the postfix service after changing any options in the configuration files in the /etc/postfix/ directory to apply those changes: Verify the email communication between local users on your system: To verify that your mail server does not relay emails from external IP ranges to foreign domains, follow the below mentioned procedure: Log in to a client which is not within the subnets that you defined in mynetworks . Configure the client to use your mail server. Try to send an email to an email address that is not under the domain you specified in mydomain on your mail server. For example, try to send an email to non-existing-user@redhat.com . Check the /var/log/maillog file: Troubleshooting In case of errors, check the /var/log/maillog file. Additional resources The /etc/postfix/main.cf configuration file The /usr/share/doc/postfix/README_FILES directory Using and configuring firewalld 2.3. 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 9, 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 If the server runs RHEL 9.2 or later and the FIPS mode is enabled, clients must either support the Extended Master Secret (EMS) extension or use TLS 1.3. TLS 1.2 connections without EMS fail. For more information, see the Red Hat Knowledgebase solution TLS extension "Extended Master Secret" enforced . Procedure 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: Restrict the incoming SMTP connections to authenticated users only by editing the /etc/postfix/main.cf file: Reload the postfix service to apply the changes: 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 : 2.4. 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 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: Configure Postfix to forward all email by setting the mydestination parameter equal to an empty value: In this configuration the Postfix server is not a destination for any email and acts as a null client. Specify the mail relay server that receives the email from your null client: The relay host is responsible for the mail delivery. Enclose <ip_address_or_hostname> in square brackets. Configure the Postfix mail server to listen only on the loopback interface for emails to deliver: If you want Postfix to rewrite the sender domain of all outgoing emails to the company domain of your relay mail server, set: To disable the local mail delivery, add the following directive at the end of the configuration file: 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: Verify if the Postfix configuration in the main.cf file is correct: Restart the postfix service to apply the changes: Verification Verify that the email communication is forwarded to the mail relay: Troubleshooting In case of errors, check the /var/log/maillog file. Additional resources The /etc/postfix/main.cf configuration file 2.5. 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 In the /etc/postfix/virtual virtual alias file, specify the email addresses for each domain. Add each email address on a new line: 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. Create a hash file for the virtual alias map: 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. In the Postfix /etc/postfix/main.cf configuration file, add the virtual_alias_maps parameter and point it to the hash file: Reload the postfix service to apply the changes: 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. 2.6. 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 Configure the LDAP lookup parameters by creating a /etc/postfix/ldap-aliases.cf file with the following content: Specify the hostname of the LDAP server: Specify the base domain name for the LDAP search: Optional: Customize the LDAP search filter and attributes based on your requirements. The filter for searching the directory defaults to query_filter = mailacceptinggeneralid=%s . Enable the LDAP source as a lookup table in the /etc/postfix/main.cf configuration file by adding the following content: Verify the LDAP configuration by running the postmap command, which checks for any syntax errors or connectivity issues: Reload the postfix service to apply the changes: 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 2.7. 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 To configure Postfix as an outgoing mail server, edit the /etc/postfix/main.cf file and add the following: Enable SMTP authentication: Disable access without TLS: Allow mail relaying only for authenticated users: Optional: Restrict users to use their own email address only as a sender: Reload the postfix service to apply the changes: Verification Authenticate in your SMTP client that supports TLS and SASL. Send an test email to verify that the SMTP authentication works correctly. 2.8. 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 You have the root access. You have configured a Postfix server. You have configured a Dovecot server, see Configuring and maintaining a Dovecot IMAP and POP3 server . You have configured the LMTP socket on your Dovecot server, see Configuring an LMTP socket and LMTPS listener . Procedure 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: If you want to use non-virtual mailboxes, add the following content: Reload postfix to apply the changes: Verification Send an test email to verify that the LMTP socket works correctly. Check the mail logs in /var/log/maillog for any errors. 2.9. 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 You have the root access. You have configured a Postfix server. You have configured a Dovecot server, see Configuring and maintaining a Dovecot IMAP and POP3 server . You have configured the LMTP service on your Dovecot server, see Configuring an LMTP socket and LMTPS listener . Procedure 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: Replace <dovecot_host> with the IP address or hostname of the Dovecot server and <port> with the port number of the LMTP service. Reload the postfix service to apply the changes: 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. 2.10. 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. 2.10.1. Reducing Postfix network-related security risks To reduce the risk of attackers invading your system through the network, perform as many of the following tasks as possible. Do not share the /var/spool/postfix/ mail spool directory on a Network File System (NFS) shared volume. NFSv2 and NFSv3 do not maintain control over user and group IDs. Therefore, if two or more users have the same UID, they can receive and read each other's mail, which is a security risk. Note This rule does not apply to NFSv4 using Kerberos, because the SECRPC_GSS kernel module does not use UID-based authentication. However, to reduce the security risks, you should not put the mail spool directory on NFS shared volumes. To reduce the probability of Postfix server exploits, mail users must access the Postfix server using an email program. Do not allow shell accounts on the mail server, and set all user shells in the /etc/passwd file to /sbin/nologin (with the possible exception of the root user). To protect Postfix from a network attack, it is set up to only listen to the local loopback address by default. You can verify this by viewing the inet_interfaces = localhost line in the /etc/postfix/main.cf file. This ensures that Postfix only accepts mail messages (such as cron job reports) from the local system and not from the network. This is the default setting and protects Postfix from a network attack. To remove the localhost restriction and allow Postfix to listen on all interfaces, set the inet_interfaces parameter to all in /etc/postfix/main.cf . 2.10.2. 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 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 Defines a time unit to calculate the rate limit. The default value is 60 seconds. smtpd_client_event_limit_exceptions Excludes clients from the connection and rate limit commands. By default, the directive excludes clients in trusted networks. smtpd_client_message_rate_limit 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 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 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 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 Defines the maximum size of a message including the envelope information in bytes. By default, the value is 10240000 bytes. 2.10.3. 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 Set up Dovecot: Include the following lines in the /etc/dovecot/conf.d/10-master.conf file: The 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. Optional: Set up Dovecot to listen for Postfix authentication requests through TCP: 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: The auth_mechanisms parameter supports different plaintext and non-plaintext authentication methods. Set up Postfix by modifying the /etc/postfix/main.cf file: Enable SMTP Authentication on the Postfix SMTP server: Enable the use of Dovecot SASL implementation for SMTP Authentication: 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: 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: In the 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. Specify SASL mechanisms that the Postfix SMTP server makes available to clients. Note that you can specify different mechanisms for encrypted and unencrypted sessions. The 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 Postfix SMTP server policy - SASL mechanism properties Postfix and Dovecot SASL Configuring SASL authentication in the Postfix SMTP server

Commands:
dnf remove sendmail
dnf install postfix
inet_interfaces = 127.0.0.1/32, [::1]/128, 192.0.2.1, [2001:db8:1::1]
inet_interfaces = all
myhostname = <smtp.example.com>
mydomain = <example.com>
myorigin = USDmydomain
mynetworks = 127.0.0.1/32, [::1]/128, 192.0.2.1/24, [2001:db8:1::1]/64
postfix check
systemctl enable --now postfix
firewall-cmd --permanent --add-service smtp firewall-cmd --reload
systemctl status postfix
systemctl restart postfix
systemctl reload postfix
echo "This is a test message" | mail -s <SUBJECT> <user@mydomain.com>
554 Relay access denied - the server is not going to relay. 250 OK or similar - the server is going to relay.
smtpd_tls_cert_file = /etc/pki/tls/certs/postfix.pem smtpd_tls_key_file = /etc/pki/tls/private/postfix.key
smtpd_tls_auth_only = yes
systemctl reload postfix
smtp_tls_loglevel = 1
mydestination =
relayhost = <[ip_address_or_hostname]>
inet_interfaces = loopback-only
myorigin = <relay.example.com>
local_transport = error: local delivery disabled
mynetworks = 127.0.0.0/8, [::1]/128
postfix check
systemctl restart postfix
echo "This is a test message" | mail -s <SUBJECT> <user@example.com>
<info@example.com> <user22@example.net> <sales@example.com> <user11@example.org>
postmap /etc/postfix/virtual
virtual_alias_maps = hash:/etc/postfix/virtual
systemctl reload postfix
server_host = <ldap.example.com>
search_base = dc= <example> ,dc= <com>
virtual_alias_maps = ldap:/etc/postfix/ldap-aliases.cf
postmap -q @ <example.com> ldap:/etc/postfix/ldap-aliases.cf
systemctl reload postfix
smtpd_sasl_auth_enable = yes broken_sasl_auth_clients = yes
smtpd_tls_auth_only = yes
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
smtpd_sender_restrictions = reject_sender_login_mismatch
systemctl reload postfix
virtual_transport = lmtp:unix:/var/run/dovecot/lmtp
mailbox_transport = lmtp:unix:/var/run/dovecot/lmtp
systemctl reload postfix
mailbox_transport = lmtp:inet: <dovecot_host> : <port>
systemctl reload postfix
service auth { unix_listener /var/spool/postfix/private/auth { mode = 0660 user = postfix group = postfix } }
service auth { inet_listener { port = port-number } }
auth_mechanisms = plain login
smtpd_sasl_auth_enable = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_path = inet: ip-address : port-number
smtpd_sasl_security_options = noanonymous, noplaintext smtpd_sasl_tls_security_options = noanonymous