# Postfix Standard Configuration Examples

## Purpose of this document

This document presents a number of typical Postfix configurations. This document should be reviewed after you have followed the basic configuration steps as described in the BASIC_CONFIGURATION_README document. In particular, do not proceed here if you don't already have Postfix working for local mail submission and for local mail delivery.

The first part of this document presents standard configurations that each solve one specific problem.

- Postfix on a stand-alone Internet host
- Postfix on a null client
- Postfix on a local network
- Postfix email firewall/gateway

The second part of this document presents additional configurations for hosts in specific environments.

- Delivering some but not all accounts locally
- Running Postfix behind a firewall
- Configuring Postfix as MX host for a remote site
- Postfix on a dialup machine
- Postfix on hosts without a real hostname

## Postfix on a stand-alone Internet host

Postfix should work out of the box without change on a stand-alone machine that has direct Internet access. At least, that is how Postfix installs when you download the Postfix source code via http://www.postfix.org/.

You can use the command "postconf -n" to find out what settings are overruled by your main.cf. Besides a few pathname settings, few parameters should be set on a stand-alone box, beyond what is covered in the BASIC_CONFIGURATION_README document:

>     /etc/postfix/main.cf:
>         # Optional: send mail as user@domainname instead of user@hostname.
>         #myorigin = $mydomain

>         # Optional: specify NAT/proxy external address.
>         #proxy_interfaces = 1.2.3.4

>         # Don't relay mail from other hosts.
>         mynetworks_style = host
>         relay_domains =

See also the section "Postfix on hosts without a real hostname" if this is applicable to your configuration.

## Postfix on a null client

A null client is a machine that can only send mail. It receives no mail from the network, and it does not deliver any mail locally. A null client typically uses POP, IMAP or NFS for mailbox access.

In this example we assume that the Internet domain name is "example.com" and that the machine is named "nullclient.example.com". As usual, the examples show only parameters that are not left at their default settings.

>     1 /etc/postfix/main.cf:
>     2     myorigin = $mydomain
>     3     relayhost = $mydomain
>     4     inet_interfaces = 127.0.0.1
>     5     local_transport = error:local delivery is disabled
>     6
>     7 /etc/postfix/master.cf:
>     8     Comment out the local delivery agent entry

Translation:

- Line 2: Send mail as "user@example.com" (instead of "user@nullclient.example.com"), so that nothing ever has a reason to send mail to "user@nullclient.example.com".

- Line 3: Forward all mail to the mail server that is responsible for the "example.com" domain. This prevents mail from getting stuck on the null client if it is turned off while some remote destination is unreachable.

- Line 4: Do not accept mail from the network.

- Lines 5-8: Disable local mail delivery. All mail goes to the mail server as specified in line 3.

## Postfix on a local network

This section describes a local area network environment of one main server and multiple other systems that send and receive email. As usual we assume that the Internet domain name is "example.com". All systems are configured to send mail as "user@example.com", and all systems receive mail for "user@hostname.example.com". The main server also receives mail for "user@example.com". We call this machine by the name of mailhost.example.com.

A drawback of sending mail as "user@example.com" is that mail for "root" and other system accounts is also sent to the central mailhost. See the section "Delivering some but not all accounts locally" below for possible solutions.

As usual, the examples show only parameters that are not left at their default settings.

First we present the non-mailhost configuration, because it is the simpler one. This machine sends mail as "user@example.com" and is final destination for "user@hostname.example.com".

>     1 /etc/postfix/main.cf:
>     2     myorigin = $mydomain
>     3     mynetworks = 127.0.0.0/8 10.0.0.0/24
>     4     relay_domains =
>     5     # Optional: forward all non-local mail to mailhost
>     6     #relayhost = $mydomain

Translation:

- Line 2: Send mail as "user@example.com".

- Line 3: Specify the trusted networks.

- Line 4: This host does not relay mail from untrusted networks.

- Line 6: This is needed if no direct Internet access is available. See also below, "Postfix behind a firewall".

Next we present the mailhost configuration. This machine sends mail as "user@example.com" and is final destination for "user@hostname.example.com" as well as "user@example.com".

>      1 DNS:
>      2     example.com    IN    MX  10 mailhost.example.com.
>      3
>      4 /etc/postfix/main.cf:
>      5     myorigin = $mydomain
>      6     mydestination = $myhostname localhost.$mydomain localhost $mydomain
>      7     mynetworks = 127.0.0.0/8 10.0.0.0/24
>      8     relay_domains =
>      9     # Optional: forward all non-local mail to firewall
>     10     #relayhost = [firewall.example.com]

Translation:

- Line 2: Send mail for the domain "example.com" to the machine mailhost.example.com. Remember to specify the "." at the end of the line.

- Line 5: Send mail as "user@example.com".

- Line 6: This host is the final mail destination for the "example.com" domain, in addition to the names of the machine itself.

- Line 7: Specify the trusted networks.

- Line 8: This host does not relay mail from untrusted networks.

- Line 10: This is needed only when the mailhost has to forward non-local mail via a mail server on a firewall. The `[]` forces Postfix to do no MX record lookups.

In an environment like this, users access their mailbox in one or more of the following ways:

- Mailbox access via NFS or equivalent.

- Mailbox access via POP or IMAP.

- Mailbox on the user's preferred machine.

In the latter case, each user has an alias on the mailhost that forwards mail to her preferred machine:

>     /etc/aliases:
>         joe:    joe@joes.preferred.machine
>         jane:   jane@janes.preferred.machine

On some systems the alias database is not in /etc/aliases. To find out the location for your system, execute the command "postconf alias_maps".

Execute the command "newaliases" whenever you change the aliases file.

## Postfix email firewall/gateway

The idea is to set up a Postfix email firewall/gateway that forwards mail for "example.com" to an inside gateway machine but rejects mail for "anything.example.com". There is only one problem: with "relay_domains = example.com", the firewall normally also accepts mail for "anything.example.com". That would not be right.

Note: this example requires Postfix version 2.0 and later. To find out what Postfix version you have, execute the command "postconf mail_version".

The solution is presented in multiple parts. This first part gets rid of local mail delivery on the firewall, making the firewall harder to break.

>     1 /etc/postfix/main.cf:
>     2     myorigin = example.com
>     3     mydestination =
>     4     local_recipient_maps =
>     5     local_transport = error:local mail delivery is disabled
>     6
>     7 /etc/postfix/master.cf:
>     8     Comment out the local delivery agent

Translation:

- Line 2: Send mail from this machine as "user@example.com", so that no reason exists to send mail to "user@firewall.example.com".

- Lines 3-8: Disable local mail delivery on the firewall machine.

For the sake of technical correctness the firewall must be able to receive mail for postmaster@[firewall ip address]. Reportedly, some things actually expect this ability to exist. The second part of the solution therefore adds support for postmaster@[firewall ip address], and as a bonus we do abuse@[firewall ip address] as well. All the mail to these two accounts is forwarded to an inside address.

>     1 /etc/postfix/main.cf:
>     2     virtual_alias_maps = hash:/etc/postfix/virtual
>     3
>     4 /etc/postfix/virtual:
>     5     postmaster      postmaster@example.com
>     6     abuse           abuse@example.com

Translation:

- Because mydestination is empty (see the previous example), only address literals matching $inet_interfaces or $proxy_interfaces are deemed local. So "localpart@[a.d.d.r]" can be matched as simply "localpart" in canonical(5) and virtual(5). This avoids the need to specify firewall IP addresses into Postfix configuration files.

The last part of the solution does the email forwarding, which is the real purpose of the firewall email function.

>      1 /etc/postfix/main.cf:
>      2     mynetworks = 127.0.0.0/8 12.34.56.0/24
>      3     relay_domains = example.com
>      4     parent_domain_matches_subdomains =
>      5         debug_peer_list smtpd_access_maps
>      6     smtpd_recipient_restrictions =
>      7         permit_mynetworks reject_unauth_destination
>      8
>      9     relay_recipient_maps = hash:/etc/postfix/relay_recipients
>     10     transport_maps = hash:/etc/postfix/transport
>     11
>     12 /etc/postfix/relay_recipients:
>     13     user1@example.com   x
>     14     user2@example.com   x
>     15      . . .
>     16
>     17 /etc/postfix/transport:
>     18     example.com   smtp:[inside-gateway.example.com]

Translation:

- Lines 1-7: Accept mail from local systems in $mynetworks, and accept mail from outside for "user@example.com" but not for "user@anything.example.com". The magic is in lines 4-5.

- Lines 9, 12-14: Define the list of valid addresses in the "example.com" domain that can receive mail from the Internet. This prevents the mail queue from filling up with undeliverable MAILER-DAEMON messages. If you can't maintain a list of valid recipients then you must specify "relay_recipient_maps =" (that is, an empty value), or you must specify an "@example.com x" wild-card in the relay_recipients table.

- Lines 10, 17-18: Route mail for "example.com" to the inside gateway machine. The `[]` forces Postfix to do no MX lookup.

Specify dbm instead of hash if your system uses dbm files instead of db. To find out what lookup tables Postfix supports, use the command "postconf -m".

Execute the command "postmap /etc/postfix/relay_recipients" whenever you change the relay_recipients table.

Execute the command "postmap /etc/postfix/transport" whenever you change the transport table.

## Delivering some but not all accounts locally

A drawback of sending mail as "user@example.com" (instead of "user@hostname.example.com") is that mail for "root" and other system accounts is also sent to the central mailhost. In order to deliver such accounts locally, you can set up virtual aliases as follows:

>     1 /etc/postfix/main.cf:
>     2     virtual_alias_maps = hash:/etc/postfix/virtual
>     3
>     4 /etc/postfix/virtual:
>     5     root     root@localhost
>     6     . . .

Translation:

- Line 5: As described in the virtual(5) manual page, the bare name "root" matches "root@site" when "site" is equal to $myorigin, when "site" is listed in $mydestination, or when it matches $inet_interfaces or $proxy_interfaces.

## Running Postfix behind a firewall

The simplest way to set up Postfix on a host behind a firewalled network is to send all mail to a gateway host, and to let that mail host take care of internal and external forwarding. Examples of that are shown in the local area network section above. A more sophisticated approach is to send only external mail to the gateway host, and to send intranet mail directly. That's what Wietse does at work.

Note: this example requires Postfix version 2.0 and later. To find out what Postfix version you have, execute the command "postconf mail_version".

The following example presents additional configuration. You need to combine this with basic configuration information as discussed the first half of this document.

>      1 /etc/postfix/main.cf:
>      2     transport_maps = hash:/etc/postfix/transport
>      3     relayhost =
>      4     # Optional for a machine that isn't "always on"
>      5     #fallback_relay = [gateway.example.com]
>      6
>      7 /etc/postfix/transport:
>      8     # Internal delivery.
>      9     example.com      :
>     10     .example.com     :
>     11     # External delivery.
>     12     *                smtp:[gateway.example.com]

Translation:

- Lines 2, 7-12: Request that intranet mail is delivered directly, and that external mail is given to a gateway. Obviously, this example assumes that the organization uses DNS MX records internally. The `[]` forces Postfix to do no MX lookup.

- Line 3: IMPORTANT: do not specify a relayhost in main.cf.

- Line 5: This prevents mail from being stuck in the queue when the machine is turned off. Postfix tries to deliver mail directly, and gives undeliverable mail to a gateway.

Specify dbm instead of hash if your system uses dbm files instead of db. To find out what lookup tables Postfix supports, use the command "postconf -m".

Execute the command "postmap /etc/postfix/transport" whenever you edit the transport table.

## Configuring Postfix as MX host for a remote site

This section presents additional configuration. You need to combine this with basic configuration information as discussed the first half of this document.

When your system is SECONDARY MX host for a remote site this is all you need:

>      1 DNS:
>      2     the.backed-up.domain.tld        IN      MX 100 your.machine.tld.
>      3
>      4 /etc/postfix/main.cf:
>      5     relay_domains = . . . the.backed-up.domain.tld
>      6     smtpd_recipient_restrictions =
>      7         permit_mynetworks reject_unauth_destination
>      8
>      9     # You must specify your NAT/proxy external address.
>     10     #proxy_interfaces = 1.2.3.4
>     11
>     12     relay_recipient_maps = hash:/etc/postfix/relay_recipients
>     13
>     14 /etc/postfix/relay_recipients:
>     15     user1@the.backed-up.domain.tld   x
>     16     user2@the.backed-up.domain.tld   x
>     17      . . .

When your system is PRIMARY MX host for a remote site you need the above, plus:

>     18 /etc/postfix/main.cf:
>     19     transport_maps = hash:/etc/postfix/transport
>     20
>     21 /etc/postfix/transport:
>     22     the.backed-up.domain.tld       relay:[their.mail.host.tld]

Important notes:

- Do not list the.backed-up.domain.tld in mydestination.

- Do not list the.backed-up.domain.tld in virtual_alias_domains.

- Do not list the.backed-up.domain.tld in virtual_mailbox_domains.

- Lines 1-7: Forward mail from the Internet for "the.backed-up.domain.tld" to the primary MX host for that domain.

- Line 10: This is a must if Postfix receives mail via a NAT relay or proxy that presents a different IP address to the world than the local machine.

- Lines 12-16: Define the list of valid addresses in the "the.backed-up.domain.tld" domain. This prevents your mail queue from filling up with undeliverable MAILER-DAEMON messages. If you can't maintain a list of valid recipients then you must specify "relay_recipient_maps =" (that is, an empty value), or you must specify an "@example.com x" wild-card in the relay_recipients table.

- Line 22: The `[]` forces Postfix to do no MX lookup.

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what lookup tables Postfix supports, use the command "postconf -m".

Execute the command "postmap /etc/postfix/transport" whenever you change the transport table.

## Postfix on a dialup machine

This section applies to dialup connections that are down most of the time. For dialup connections that are up 24x7, see the local area network section above.

This section presents additional configuration. You need to combine this with basic configuration information as discussed the first half of this document.

If you do not have your own hostname (as with dynamic IP addressing) then you should also study the section on "Postfix on hosts without a real hostname".

- Route all outgoing mail to your network provider.

  If your machine is disconnected most of the time, there isn't a lot of opportunity for Postfix to deliver mail to hard-to-reach corners of the Internet. It's better to give the mail to a machine that is connected all the time. In the example below, the `[]` prevents Postfix from trying to look up DNS MX records.

      /etc/postfix/main.cf:
          relayhost = [smtprelay.someprovider.com]

- Disable spontaneous SMTP mail delivery (if using on-demand dialup IP only).

  Normally, Postfix attempts to deliver outbound mail at its convenience. If your machine uses on-demand dialup IP, this causes your system to place a telephone call whenever you submit new mail, and whenever Postfix retries to deliver delayed mail. To prevent such telephone calls from being placed, disable spontaneous SMTP mail deliveries.

      /etc/postfix/main.cf:
          defer_transports = smtp (Only for on-demand dialup IP hosts)

- Disable SMTP client DNS lookups (dialup LAN only).

      /etc/postfix/main.cf:
          disable_dns_lookups = yes (Only for on-demand dialup IP hosts)

- Flush the mail queue whenever the Internet link is established.

  Put the following command into your PPP or SLIP dialup scripts:

      /usr/sbin/sendmail -q (whenever the Internet link is up)

  The exact location of the sendmail command is system-specific. Use the command "postconf sendmail_path" to find out where the Postfix sendmail command is located on your machine.

  In order to find out if the mail queue is flushed, use something like:

      #!/bin/sh

      # Start mail deliveries.
      /usr/sbin/sendmail -q

      # Allow deliveries to start.
      sleep 10

      # Loop until all messages have been tried at least once.
      while mailq | grep '^[^ ]*\*' >/dev/null
      do
          sleep 10
      done

  If you have disabled spontaneous SMTP mail delivery, you also need to run the "sendmail -q" command every now and then while the dialup link is up, so that newly-posted mail is flushed from the queue.

## Postfix on hosts without a real hostname

This section is for hosts that don't have an Internet hostname. Typically these are systems that get a dynamic IP address via DHCP or via dialup. Postfix will let you send and receive mail just fine between accounts on a machine with a fantasy name. However, you cannot use a fantasy hostname in your email address when sending mail into the Internet, because no-one would be able to reply to your mail. In fact, more and more sites refuse mail from non-existent domain names.

The perfect solution would be for Postfix to do a mapping from local fantasy email addresses to valid Internet addresses when mail leaves the machine (similar to Sendmail's generics table). This is planned for the near future.

In the mean time, the solution with Postfix is to use valid Internet addresses where possible, and to let Postfix map valid Internet addresses to local fantasy addresses. With this, you can send mail to the Internet and to local fantasy addresses, including mail to local fantasy addresses that don't have a valid Internet address of their own.

The following example presents additional configuration. You need to combine this with basic configuration information as discussed the first half of this document.

>      1 /etc/postfix/main.cf:
>      2     myhostname = hostname.localdomain
>      3     mydomain = localdomain
>      4
>      5     canonical_maps = hash:/etc/postfix/canonical
>      6
>      7     virtual_alias_maps = hash:/etc/postfix/virtual
>      8
>      9 /etc/postfix/canonical:
>     10     your-login-name    your-account@your-isp.com
>     11
>     12 /etc/postfix/virtual:
>     13     your-account@your-isp.com       your-login-name

Translation:

- Lines 2-3: Substitute your fantasy hostname here. Do not use a domain name that is already in use by real organizations on the Internet. See RFC 2606 for examples of domain names that are guaranteed not to be owned by anyone.

- Lines 5, 9, 10: This provides the mapping from "your-login-name@hostname.localdomain" to "your-account@your-isp.com". This part is required.

- Lines 7, 12, 13: Deliver mail for "your-account@your-isp.com" locally, instead of sending it to the ISP. This part is not required but is convenient.