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. 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.  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:

sender_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