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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Address Classes </h1>

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<h2>Introduction</h2>

<p> 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. </p>

<p> This document provides information on the following topics: </p>

<ul>

<li><a href="#wtf">What are address classes good for?</a>

<li><a href="#classes">What address classes does Postfix implement?</a>

<li><a href="#improvements">Improvements compared to Postfix 1.1</a>

<li><a href="#incompatibility">Incompatibilities with Postfix 1.1</a>

</ul>

<h2><a name="wtf">What are address classes good for?</a></h2>

<p> 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. </p>

<p> An address class is defined by three items. </p>

<ul>

<li> <p> The list of domains that are a member of that address
class. </p>

<p> Examples: all local domains, or all relay domains. </p>

<li> <p> The default delivery transport for domains in that address
class. </p>

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

<p> Benefit: this avoids the need for explicit routing information
in transport maps. </p>

<li> <p> The list of valid recipient addresses for that address
class. </p>

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

</ul>

<h2><a name="classes">What address classes does Postfix implement?</a></h2>

<p> 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. </p>

<p> The <a name="local_domain_class">local </a> domain class. </p>

<ul>

<li> <p> 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. </p>

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

<li> <p> 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. </p>

<li> <p> The mail delivery transport is specified with the
local_transport parameter. The default value is <b>local:$myhostname</b>
for delivery with the local(8) delivery agent. </p>

</ul>

<p> The <a name="virtual_alias_class">virtual alias </a> domain
class. </p>

<ul>

<li> <p> 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. </p>

<li> <p> Domain names are listed in virtual_alias_domains. The
default value is $virtual_alias_maps for Postfix 1.1 compatibility.
</p>

<li> <p> 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. </p>

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

<li> <p> There is no configurable mail delivery transport. Every
address must be aliased to an address in some other domain class. </p>

</ul>

<p> The <a name="virtual_mailbox_class">virtual mailbox </a> domain
class. </p>

<ul>

<li> <p> 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. </p>

<li> <p> Domain names are listed with the virtual_mailbox_domains
parameter. The default value is $virtual_mailbox_maps for Postfix
1.1 compatibility. </p>

<li> <p> 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.
</p>

<li> <p> The mail delivery transport is specified with the
virtual_transport parameter. The default value is <b>virtual</b>
for delivery with the virtual(8) delivery agent. </p>

</ul>

<p> The <a name="relay_domain_class">relay </a> domain class. </p>

<ul>

<li> <p> 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. </p>

<li> <p> Domain names are listed with the relay_domains parameter.
</p>

<li> <p> 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. </p>

<li> <p> The mail delivery transport is specified with the
relay_transport parameter. The default value is <b>relay</b> which
is a clone of the smtp(8) delivery agent. </p>

</ul>

<p> The <a name="default_domain_class">default </a> domain class.
</p>

<ul>

<li> <p> 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. </p>

<li> <p> This class has no destination domain table. </p>

<li> <p> This class has no valid recipient address table. </p>

<li> <p> The mail delivery transport is specified with the
default_transport parameter. The default value is <b>smtp</b> for
delivery with the smtp(8) delivery agent. </p>

</ul>

<h2><a name="improvements">Improvements compared to Postfix
1.1</a></h2>

<p> Postfix 2.0 address classes made the following improvements
possible over earlier Postfix versions: </p>

<ul>

<li> <p> 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).
</p>

<li> <p> 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. </p>

<li> <p> 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. </p>

<li> <p> 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. </p>

</ul>

<h2><a name="incompatibility">Incompatibilities with Postfix 1.1</a></h2>

<p> 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. </p>

<ul>

<li> <p> 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"). </p>

<p> 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.
</p>

<li> <p> 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. </p>

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

<li> <p> 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. </p>

<li> <p> 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. </p>

<li> <p> 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. </p>

</ul>

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