# Postfix Address Classes

## Introduction

Postfix version 2.0 introduces the concept of address classes. This is a way of grouping recipient addresses by their delivery method. The idea comes from discussions with Victor Duchovni. Although address classes introduced a few incompatibilities they also made it possible to improve the handling of hosted domains and of unknown recipients.

This document provides information on the following topics:

- What are address classes good for?
- What address classes does Postfix implement?
- Improvements compared to Postfix 1.1
- Incompatibilities with Postfix 1.1

## What are address classes good for?

Why should you care about address classes? This is how Postfix decides what mail to accept, and how to deliver it. In other words, address classes are very important for the operation of Postfix.

An address class is defined by three items.

- The list of domains that are a member of that address class.

  Examples: all local domains, or all relay domains.

- The default delivery transport for domains in that address class.

  Examples: local_transport or relay_transport (these point to services defined in master.cf).

  Benefit: this avoids the need for explicit routing information in transport maps.

- The list of valid recipient addresses for that address class.

  Benefit: the Postfix SMTP server rejects an invalid recipient with "User unknown in <name of address class> table", and avoids sending a MAILER-DAEMON message with backscatter spam.

## What address classes does Postfix implement?

Initially the list of address classes is hard coded, but this is meant to become extensible. The summary below describes the main purpose of each class, and what the relevant configuration parameters are.

The local domain class.

- Purpose: final delivery for traditional UNIX system accounts and traditional Sendmail-style aliases. This is typically used for the canonical domains of the machine (for example, $myhostname, $mydomain). For a discussion of the difference between canonical domains, hosted domains and other domains, see the VIRTUAL_README file.

- Domain names are listed with the mydestination parameter. This domain class also includes mail for user@[ipaddress] when the IP address is listed with the inet_interfaces or proxy_interfaces parameters.

- Valid recipient addresses for those domains are listed with the local_recipient_maps parameter, as described in LOCAL_RECIPIENT_README. The Postfix SMTP server rejects invalid recipients with "User unknown in local recipient table". If the local_recipient_maps parameter value is empty, then the Postfix SMTP server accepts any address in the local domain class.

- The mail delivery transport is specified with the local_transport parameter. The default value is local:$myhostname for delivery with the local(8) delivery agent.

The virtual alias domain class.

- Purpose: hosted domains where each recipient address is aliased to an address in a different domain class, for example, a local UNIX system account or a remote address. A virtual alias example is given in the VIRTUAL_README file.

- Domain names are listed in virtual_alias_domains. The default value is $virtual_alias_maps for Postfix 1.1 compatibility.

- Valid recipient addresses for those domains are listed with the virtual_alias_maps parameter. The Postfix SMTP server rejects invalid recipients with "User unknown in virtual alias table". The default value is $virtual_maps for Postfix 1.1 compatibility.

  > Note: for historical reasons, virtual_alias_maps apply to recipients in all domain classes, not only the virtual alias domain class.

- There is no configurable mail delivery transport. Every address must be aliased to an address in some other domain class.

The virtual mailbox domain class.

- Purpose: final delivery for hosted domains where each recipient address can have its own mailbox, and where users do not need to have a UNIX system account. A virtual mailbox example is given in the VIRTUAL_README file.

- Domain names are listed with the virtual_mailbox_domains parameter. The default value is $virtual_mailbox_maps for Postfix 1.1 compatibility.

- Valid recipient addresses for those domains are listed with the virtual_mailbox_maps parameter. The Postfix SMTP server rejects invalid recipients with "User unknown in virtual mailbox table". If this parameter value is empty, the Postfix SMTP server accepts all recipients for domains listed in $virtual_mailbox_domains.

- The mail delivery transport is specified with the virtual_transport parameter. The default value is virtual for delivery with the virtual(8) delivery agent.

The relay domain class.

- Purpose: mail forwarding to remote destinations that list your system as primary or backup MX host. For a discussion of the basic configuration details, see the BASIC_CONFIGURATION_README document. For a discussion of the difference between canonical domains, hosted domains and other domains, see the VIRTUAL_README file.

- Domain names are listed with the relay_domains parameter.

- Valid recipient addresses for those domains are listed with the relay_recipient_maps parameter. The Postfix SMTP server rejects invalid recipients with "User unknown in relay recipient table". If this parameter value is empty, the Postfix SMTP server accepts all recipients for domains listed with the relay_domains parameter.

- The mail delivery transport is specified with the relay_transport parameter. The default value is relay which is a clone of the smtp(8) delivery agent.

The default domain class.

- Purpose: mail forwarding to the Internet on behalf of authorized clients. For a discussion of the basic configuration details, see the BASIC_CONFIGURATION_README file. For a discussion of the difference between canonical domains, hosted domains and other domains, see the VIRTUAL_README file.

- This class has no destination domain table.

- This class has no valid recipient address table.

- The mail delivery transport is specified with the default_transport parameter. The default value is smtp for delivery with the smtp(8) delivery agent.

## Improvements compared to Postfix 1.1

Postfix 2.0 address classes made the following improvements possible over earlier Postfix versions:

- You no longer need to specify all the virtual(8) mailbox domains in the Postfix transport map. The virtual(8) delivery agent has become a first-class citizen just like local(8) or smtp(8).

- On mail gateway systems, address classes provide separation of inbound mail relay traffic ($relay_transport) from outbound traffic ($default_transport). This eliminates a problem where inbound mail deliveries could become resource starved in the presence of a high volume of outbound mail.

- The SMTP server rejects unknown recipients in a more consistent manner than was possible with Postfix version 1. This is needed to keep undeliverable mail (and bounced undeliverable mail) out of the mail queue. This is controlled by the smtpd_reject_unlisted_recipient configuration parameter.

- As of Postfix version 2.1, the SMTP server can also reject unknown sender addresses (i.e. addresses that it would reject as an unknown recipient addresses). Sender "egress filtering" can help to slow down an email worm explosion. This is controlled by the smtpd_reject_unlisted_sender configuration parameter.

## Incompatibilities with Postfix 1.1

Postfix 2.0 address classes introduce a few incompatible changes in documented behavior. In order to ease the transitions, new parameters have default values that are backwards compatible.

- The virtual_maps parameter is replaced by virtual_alias_maps (for address lookups) and by virtual_alias_domains (for the names of what were formerly called "Postfix-style virtual domains").

  For backwards compatibility with Postfix version 1.1, the new virtual_alias_maps parameter defaults to $virtual_maps, and the new virtual_alias_domains parameter defaults to $virtual_alias_maps.

- The virtual_mailbox_maps parameter now has a companion parameter called virtual_mailbox_domains (for the names of domains served by the virtual delivery agent). The virtual_mailbox_maps parameter is now used for address lookups only.

  For backwards compatibility with Postfix version 1.1, the new virtual_mailbox_domains parameter defaults to $virtual_mailbox_maps.

- Introduction of the relay_recipient_maps parameter. The Postfix SMTP server can use this to block mail for relay recipients that don't exist. This list is empty by default, which means accept any recipient.

- The local_recipient_maps feature is now turned on by default. The Postfix SMTP server uses this to reject mail for unknown local recipients. See the LOCAL_RECIPIENT_README file hints and tips.

- Introduction of the relay delivery transport in master.cf. This helps to avoid mail delivery scheduling problems on inbound mail relays when there is a lot of outbound mail, but may require that you update your "defer_transports" setting.