The upstream postfix documentation is detailed and well maintained. It is provided in the postfix-doc package. This README supplements the upstream documentation and provides information on Debian specific differences. In order to support multiple postfix instances, postfix uses multiple systemd unit files. The overall postfix unit is for overall operations on all instances and individual unit files are available for each instance named postfix@${INSTANCE_NAME}. The primary instance is named "-", so it can be directly addressed with systemctl using the name postfix@- (e.g. systemctl status postfix@-). Wild cards work and so systemctl status postfix* will show the status of all active postfix units. In order to configure multiple postfix instances, follow the upstream directions (MULTI_INSTANCE_README) up to the point of starting the new instance. Instead do the following (if running systemd - multi-instance with sysv init is untested by the maintainers in Debian 9, code name stretch): # systemctl daemon-reload # systemctl enable postfix@${INSTANCE_NAME}.service # systemctl start postfix@${INSTANCE_NAME}.service Once that is done, the new instance will be started and its status will show up with as part of systemctl status postfix*. There are some significant differences between the Debian Postfix packages, and the source from upstream: 1. The Debian install is chrooted by default. 2. Debian init system (systemd or sysv init) commands (e.g. systemctl or service) should be used in lieu of direct calls to the postfix binary as described in the upstream documentation is order to problem integrate with Debian features such as using the system CA certificate bundle and proper chroot configuration with system libraries and services. 2A. Starting with Debian Bookworm (12), user specified files can be included in the chroot when postfix starts. The file /etc/default/postfix is sourced into the Debian script that configures the chroot for each postfix instance. In the 'default' file, extra files can be specified in the chroot_extra_files variable (quoted, space separated list for multiple files) and an additional Certificate Authority directory can be specified in the chroot_extra_CAdir variable, for example: chroot_extra_files=/etc/ssl/certs/local-certificate.pem chroot_extra_CAdir=/etc/ssl/certs/local-CA This can be used to provide certificates needed for Postfix to make encrypted LDAP connections, e.g. tls_ca_cert_dir or tls_cert. See ldap_table (5) or mysql_table (5) for specifics. The files or directory must exist outside the chroot in the location specified. They will be created or updated within the chroot each time Postfix is started or restarted. Reload is not sufficient. 2B. In the standard Debian networking configuration, postfix is not notified if /etc/resolv.conf is updated, so the copy in the postfix chroot may become stale. This can be addressed one of two ways: a. For systemd users, a .path unit is shipped disabled (since this is only relevant for some network configurations) to watch for resolv.conf updates. It needs to be manually enabled and started (once): # systemctl enable postfix-resolvconf.path # systemctl enable postfix-resolvconf.service # systemctl start postfix-resolvconf.path b. For users of other init systems, installation of the resolvconf package should prevent this problem for networking configurations where it is an issue. 3. For policy reasons: a. SASL configuration goes in /etc/postfix/sasl b. myhostname=/path/to/file is supported (and used) in main.cf 4. IPV6 support is enabled: postfix listens on ipv6/ipv4 by default, (see: inet_protocols) 5. TLS/SASL support is enabled. 6. rmail comes from sendmail, not from postfix. 7. The upstream main.cf is delivered as /usr/share/postfix/main.cf.dist, rather than cluttering /etc/postfix/main.cf with comments. Known caveats: 1. The dynamically loadable modules are not found in the chroot. Therefore, proxy maps may require you to copy the appropriate shared object into the chroot if you chroot the proxy service in master.cf. 2. Some map types (and SASL support) require some extra configuration (beyond what upstream indicates) to run inside the chroot. The simplest solution for the maps is to use the proxy service, which is not chrooted. SASL is a bit more complex, and is on the TODO list... 3. Note that the chrooted daemons open /dev/log before chrooting, so if your syslog daemon is restarted, the daemons will be unable to reconnect to the syslog socket, and hence being unable to log. The postfix package provides a config snipped for the rsyslog daemon in /etc/rsyslog.d/postfix.conf to also open a socket in /var/log/postfix/dev. For other syslog daemons, you will also have to restart postfix after restarting the syslog daemon, or configure it to open an additional socket. a. For sysklogd (the default in Debian versions prior to Lenny), add SYSLOG="-a /var/spool/postfix/dev/log" to /etc/default/syslog. b. For inetutils-syslogd, add SYSLOGD_OPTS="-a /var/spool/postfix/dev/log" to /etc/default/inetutils-syslogd. 4. Map types from the dynamically loadable modules are supported for the alias database, but it is up to the system administrator to ensure the required package is installed before changing the postfix configuration. After changing the map type, newaliases must be run by hand. Upgrade notes: milter_protocol: Nearly all milter packages in the Debian archive use the libmilter1.0.1 library which as of version 8.14 supports sendmail milter protocol version 6. The postfix default, starting in version 2.6, is 'milter_protocol = 6'. If you are migrating a older postfix configuration that specifies a lower version, it should be safe to remove and depend on the default. If you are using milters not provided by Debian, you may need to ensure compatibility. For more information please see http://www.postfix.org/MILTER_README.html Postfix Smarthost Configuration Postfix can be configured to relay mail to a 'smarthost' for delivery. In practice, with real world smarthosts, considerable configuration is required to make this work. Some of this configuration can be done via debconf ('dpkg-reconfigure postfix'), but much of it will usually need to be done manually. This document provides instructions for such configuration. 1. Set the smarthost This can be set via debconf. To do it manually, add a line like the following to /etc/postfix/main.cf: relayhost = [relayhost.example.com]:465 If the port number is omitted, the default is 25. Most smarthosts use TLS/SSL, and accordingly generally use either 465 or 587 - see below. 2. Enable TLS/SSL As above, most smarthosts use TLS/SSL. To configure Postfix to use TLS, add the following lines to main.cf: smtp_tls_security_level = verify smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt If 'encrypt' is used instead of 'verify', the second line may be omitted. 'encrypt' means that TLS will be used but Postfix will not verify the smarthost's certificate, potentially allowing a man-in-the-middle attack and the stealing of the smarthost authentication credentials. On the other hand, 'secure' may be used as an even stricter value than 'verify'. See the explanation of 'smtp_tls_security_level' values in postconf(5) for details. If SMTPS (sometimes called 'SSL', usually used in conjunction with port 465) is desired, add the following additional line to main.cf: smtp_tls_wrappermode = yes For STARTTLS (usually used in conjunction with port 587), omit this line (or use the value 'no'). As to which port number / TLS type to use: consult your smarthost's documentation. If only one option is available, you will have to use that one. If both are available, the question is a toss-up. For the last couple of decades, STARTTLS on port 587 has been the official, standards compliant method, although SMTPS on port 465 was also widely used. Recently, RFC 8314 has proposed the official recognition of TLS on port 465. One potential weakness of STARTTLS is that as a form of opportunistic TLS, it is subject to a man-in-the-middle downgrade attack, where the server's advertisement of STARTTLS support is stripped out (STRIPTLS) by an attacker, causing the connection to continue without TLS: https://en.wikipedia.org/wiki/STARTTLS#Weaknesses_and_mitigations This can be avoided by making TLS mandatory, via the use of an appropriate value for 'smtp_tls_security_level' such as 'encrypt', 'verify', or 'secure'. 3. Configure authentication Most smarthosts require authentication. To enable it, ensure that the package 'libsasl2-modules' is installed, and add the following lines to main.cf: smtp_sasl_auth_enable = yes smtp_sasl_security_options = [See postconf(5) for more information about 'smtp_sasl_security_options' and its possible values. The above version, with no options, is generally fine.] To specify the authentication credentials, create an arbitrarily named file (e.g., '/etc/postfix/example-passwd'), with appropriately restrictive permissions (e.g., 600) containing a single line of the following form: relayhost.example.com username@example.com:secret_password Where 'relayhost.example.com' is the name of the smarthost, 'username@example.com' is the login name, and 'secret_password' is the login password. After creating the file, run the command: postmap /etc/postfix/example-passwd and add the following line to main.cf: smtp_sasl_password_maps = hash:/etc/postfix/example-passwd 4. Address rewriting Most smarthosts require that the sender (envelope FROM and perhaps also the email From: header) be set to the user's correct mail address with the smarthost. Postfix therefore needs to be configured to rewrite the sender address accordingly. There are multiple ways to do this, including canonical mapping and SMTP generic mapping. 4a. Canonical mapping With sender canonical mapping, all sender addresses are rewritten upon Postfix's receipt of the mail. Create an arbitrarily named file (e.g., '/etc/postfix/sender_canonical'), containing lines of the form local-user1 username@example.com local-user2 username@example.com where 'local-user1' and 'local-user2' are usernames on the system that will be sending mail via the smarthost After creating the file, run the command: postmap /etc/postfix/sender_canonical and add the following line to main.cf: sender_canonical_maps = hash:/etc/postfix/sender_canonical To use regular expressions to match multiple users, use either 'regexp' or 'pcre' (requires the installation of 'postfix-pcre') tables. See DATABASE_README, regexp_table(5), PCRE_README, pcre_table(5), and postmap(1). 4b. SMTP generic mapping With SMTP generic mapping, all matching addresses are rewritten upon Postfix's delivery of the mail via SMTP. Create an arbitrarily named file (e.g., '/etc/postfix/generic_mapping'), containing a line of the form: @host.domain username@example.com with 'host.domain' taken from '/etc/mailname'. After creating the file, run the command: postmap /etc/postfix/generic_mapping and add the following line to main.cf: smtp_generic_maps = hash:/etc/postfix/generic_mapping One advantage to using generic over canonical mapping is that the latter will be applied to local mail as well. If the system will be configured to send all mail, even mail addressed to local users, via the smarthost (e.g., via aliases), then this point is moot. Some mail services can be quite picky about what form of the email header From: they accept. It may be necessary to use an additional smtp_header_check rule to rewrite the header From: (whether created by the original sender, or by Postfix itself) into a form that the mail provider will accept. See: https://marc.info/?l=postfix-users&m=154662599103646 https://marc.info/?l=postfix-users&m=154656149717210 See the ADDRESS_REWRITING_README for more information. At this point, restart Postfix: /etc/init.d/postfix restart Test: echo 'test' | sendmail someuser@somehost.com 5. Aliases As configured so far, local mail will be delivered locally and not sent via the smarthost. To redirect local mail through the smarthost, aliases can be used. In /etc/aliases, add lines like the following: root: someuser@somehost.com Then run: newaliases 6. CREDITS: This guide was based (with considerable elaboration) on a number of other guides on this topic (in addition to the official Postfix documentation), including: https://www.eanderalx.org/linux/postfix http://emanuelesantanche.com/article/85/configuring-postfix-to-relay-email-through-zoho-mail https://www.dnsexit.com/support/mailrelay/postfix.html https://www.cyberciti.biz/faq/postfix-smtp-authentication-for-mail-servers/ https://blog.bravi.org/?p=1065