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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Standard Configuration Examples</h1>
<hr>
<h2>Purpose of this document</h2>
<p> 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. </p>
<p> The first part of this document presents standard configurations
that each solve one specific problem. </p>
<ul>
<li><a href="#stand_alone">Postfix on a stand-alone Internet host</a>
<li><a href="#null_client">Postfix on a null client</a>
<li><a href="#local_network">Postfix on a local network</a>
<li><a href="#firewall">Postfix email firewall/gateway</a>
</ul>
<p> The second part of this document presents additional configurations
for hosts in specific environments. </p>
<ul>
<li><a href="#some_local">Delivering some but not all accounts locally</a>
<li><a href="#intranet">Running Postfix behind a firewall</a>
<li><a href="#backup">Configuring Postfix as primary or backup MX host for a remote
site</a>
<li><a href="#dialup">Postfix on a dialup machine</a>
<li><a href="#fantasy">Postfix on hosts without a real
Internet hostname</a>
</ul>
<h2><a name="stand_alone">Postfix on a stand-alone Internet host</a></h2>
<p> 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/. </p>
<p> You can use the command "<b>postconf -n</b>" 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: </p>
<blockquote>
<pre>
/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
# Alternative 1: don't relay mail from other hosts.
mynetworks_style = host
relay_domains =
# Alternative 2: relay mail from local clients only.
# mynetworks = 192.168.1.0/28
# relay_domains =
</pre>
</blockquote>
<p> See also the section "<a href="#fantasy">Postfix on hosts without
a real Internet hostname</a>" if this is applicable to your configuration.
</p>
<h2><a name="null_client">Postfix on a null client</a></h2>
<p> 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. </p>
<p> 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. </p>
<blockquote>
<pre>
1 /etc/postfix/main.cf:
2 myorigin = $mydomain
3 relayhost = $mydomain
4 inet_interfaces = loopback-only
5 local_transport = error:local delivery is disabled
6
7 /etc/postfix/master.cf:
8 Comment out the local delivery agent entry
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> 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". </p>
<li> <p> 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. </p>
<li> <p> Line 4: Do not accept mail from the network. </p>
<li> <p> Lines 5-8: Disable local mail delivery. All mail goes to
the mail server as specified in line 3. </p>
</ul>
<h2><a name="local_network">Postfix on a local network</a></h2>
<p> 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. </p>
<p> 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 "<a href="#some_local">Delivering some
but not all accounts locally</a>" below for possible solutions.
</p>
<p> As usual, the examples show only parameters that are not left
at their default settings. </p>
<p> 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". </p>
<blockquote>
<pre>
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
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> Line 2: Send mail as "user@example.com". </p>
<li> <p> Line 3: Specify the trusted networks. </p>
<li> <p> Line 4: This host does not relay mail from untrusted networks. </p>
<li> <p> Line 6: This is needed if no direct Internet access is
available. See also below, "<a href="#firewall">Postfix behind
a firewall</a>". </p>
</ul>
<p> 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". </p>
<blockquote>
<pre>
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]
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> 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. </p>
<li> <p> Line 5: Send mail as "user@example.com". </p>
<li> <p> Line 6: This host is the final mail destination for the
"example.com" domain, in addition to the names of the machine
itself. </p>
<li> <p> Line 7: Specify the trusted networks. </p>
<li> <p> Line 8: This host does not relay mail from untrusted networks. </p>
<li> <p> Line 10: This is needed only when the mailhost has to
forward non-local mail via a mail server on a firewall. The
<tt>[]</tt> forces Postfix to do no MX record lookups. </p>
</ul>
<p> In an environment like this, users access their mailbox in one
or more of the following ways:
<ul>
<li> <p> Mailbox access via NFS or equivalent. </p>
<li> <p> Mailbox access via POP or IMAP. </p>
<li> <p> Mailbox on the user's preferred machine. </p>
</ul>
<p> In the latter case, each user has an alias on the mailhost that
forwards mail to her preferred machine: </p>
<blockquote>
<pre>
/etc/aliases:
joe: joe@joes.preferred.machine
jane: jane@janes.preferred.machine
</pre>
</blockquote>
<p> On some systems the alias database is not in /etc/aliases. To
find out the location for your system, execute the command "<b>postconf
alias_maps</b>". </p>
<p> Execute the command "<b>newaliases</b>" whenever you change
the aliases file. </p>
<h2><a name="firewall">Postfix email firewall/gateway</a></h2>
<p> 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.
</p>
<p> Note: this example requires Postfix version 2.0 and later. To find
out what Postfix version you have, execute the command "<b>postconf
mail_version</b>". </p>
<p> 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. </p>
<blockquote>
<pre>
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
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> Line 2: Send mail from this machine as "user@example.com",
so that no reason exists to send mail to "user@firewall.example.com".
</p>
<li> <p> Lines 3-8: Disable local mail delivery on the firewall
machine. </p>
</ul>
<p> 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.
</p>
<blockquote>
<pre>
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
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> 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. </p>
</ul>
<p> The last part of the solution does the email forwarding, which
is the real purpose of the firewall email function. </p>
<blockquote>
<pre>
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]
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li><p> 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. </p>
<li> <p> 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. </p>
<li> <p> Lines 10, 17-18: Route mail for "example.com" to the inside
gateway machine. The <tt>[]</tt> forces Postfix to do no MX lookup.
</p>
</ul>
<p>Specify <b>dbm</b> instead of <b>hash</b> if your system uses
<b>dbm</b> files instead of <b>db</b> files. To find out what lookup
tables Postfix supports, use the command "<b>postconf -m</b>". </p>
<p> Execute the command "<b>postmap /etc/postfix/relay_recipients</b>"
whenever you change the relay_recipients table. </p>
<p> Execute the command "<b>postmap /etc/postfix/transport</b>"
whenever you change the transport table. </p>
<p> In some installations, there may be separate instances of Postfix
processing inbound and outbound mail on a multi-homed firewall. The
inbound Postfix instance has an SMTP server listening on the external
firewall interface, and the outbound Postfix instance has an SMTP server
listening on the internal interface. In such a configuration is it is
tempting to configure $inet_interfaces in each instance with just the
corresponding interface address. </p>
<p> In most cases, using inet_interfaces in this way will not work,
because as documented in the $inet_interfaces reference manual, the
smtp(8) delivery agent will also use the specified interface address
as the source address for outbound connections and will be unable to
reach hosts on "the other side" of the firewall. The symptoms are that
the firewall is unable to connect to hosts that are in fact up. See the
inet_interfaces parameter documentation for suggested work-arounds.</p>
<h2><a name="some_local">Delivering some but not all accounts
locally</a></h2>
<p> 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: </p>
<blockquote>
<pre>
1 /etc/postfix/main.cf:
2 virtual_alias_maps = hash:/etc/postfix/virtual
3
4 /etc/postfix/virtual:
5 root root@localhost
6 . . .
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> 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. </p>
</ul>
<p> Execute the command "<b>postmap /etc/postfix/virtual</b>" after
editing the file. </p>
<h2><a name="intranet">Running Postfix behind a firewall</a></h2>
<p> 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 <a href="#local_network">local area network</a>
section above. A more sophisticated approach is to send only external
mail to the gateway host, and to send intranet mail directly. </p>
<p> Note: this example requires Postfix version 2.0 and later. To find
out what Postfix version you have, execute the command "<b>postconf
mail_version</b>". </p>
<p> The following example presents additional configuration. You
need to combine this with basic configuration information as
discussed the first half of this document. </p>
<blockquote>
<pre>
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]
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> 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 <tt>[]</tt> forces Postfix to do no MX lookup.
</p>
<li> <p> Line 3: IMPORTANT: do not specify a relayhost in main.cf.
</p>
<li> <p> 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. </p>
</ul>
<p> Specify <b>dbm</b> instead of <b>hash</b> if your system uses
<b>dbm</b> files instead of <b>db</b> files. To find out what lookup
tables Postfix supports, use the command "<b>postconf -m</b>". </p>
<p> Execute the command "<b>postmap /etc/postfix/transport</b>" whenever
you edit the transport table. </p>
<h2><a name="backup">Configuring Postfix as primary or backup MX host for a remote site</a></h2>
<p> This section presents additional configuration. You need to
combine this with basic configuration information as discussed the
first half of this document. </p>
<p> When your system is SECONDARY MX host for a remote site this
is all you need: </p>
<blockquote>
<pre>
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 . . .
</pre>
</blockquote>
<p> When your system is PRIMARY MX host for a remote site you
need the above, plus: </p>
<blockquote>
<pre>
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]
</pre>
</blockquote>
<p> Important notes:
<ul>
<li><p>Do not list the.backed-up.domain.tld in mydestination.</p>
<li><p>Do not list the.backed-up.domain.tld in virtual_alias_domains.</p>
<li><p>Do not list the.backed-up.domain.tld in virtual_mailbox_domains.</p>
<li> <p> Lines 1-7: Forward mail from the Internet for
"the.backed-up.domain.tld" to the primary MX host for that domain.
</p>
<li> <p> 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. </p>
<li> <p> 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 "@the.backed-up.domain.tld x" wild-card in the
relay_recipients table. </p>
<li> <p> Line 22: The <tt>[]</tt> forces Postfix to do no MX lookup. </p>
</ul>
<p> Specify <b>dbm</b> instead of <b>hash</b> if your system uses
<b>dbm</b> files instead of <b>db</b> files. To find out what lookup
tables Postfix supports, use the command "<b>postconf -m</b>". </p>
<p> Execute the command "<b>postmap /etc/postfix/transport</b>"
whenever you change the transport table. </p>
<p> NOTE for Postfix < 2.2: Do not use the fallback_relay feature
when relaying mail
for a backup or primary MX domain. Mail would loop between the
Postfix MX host and the fallback_relay host when the final destination
is unavailable. </p>
<ul>
<li> In main.cf specify "<tt>relay_transport = relay</tt>",
<li> In master.cf specify "<tt>-o fallback_relay =</tt>" at the
end of the <tt>relay</tt> entry.
<li> In transport maps, specify "<tt>relay:<i>nexthop...</i></tt>"
as the right-hand side for backup or primary MX domain entries.
</ul>
<p> These are default settings in Postfix version 2.2 and later.
</p>
<h2><a name="dialup">Postfix on a dialup machine</a></h2>
<p> This section applies to dialup connections that are down most
of the time. For dialup connections that are up 24x7, see the <a
href="#local_network">local area network</a> section above. </p>
<p> This section presents additional configuration. You need to
combine this with basic configuration information as discussed the
first half of this document. </p>
<p> If you do not have your own hostname and IP address (usually
with dialup, cable TV or DSL connections) then you should also
study the section on "<a href="#fantasy">Postfix on hosts without
a real Internet hostname</a>". </p>
<ul>
<li> Route all outgoing mail to your network provider.
<p> 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 <tt>[]</tt>
prevents Postfix from trying to look up DNS MX records. </p>
<pre>
/etc/postfix/main.cf:
relayhost = [smtprelay.someprovider.com]
</pre>
<li> <p><a name="spontaneous_smtp">Disable spontaneous SMTP mail
delivery (if using on-demand dialup IP only).</a> </p>
<p> 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. </p>
<pre>
/etc/postfix/main.cf:
defer_transports = smtp (Only for on-demand dialup IP hosts)
</pre>
<li> <p>Disable SMTP client DNS lookups (dialup LAN only).</p>
<pre>
/etc/postfix/main.cf:
disable_dns_lookups = yes (Only for on-demand dialup IP hosts)
</pre>
<li> Flush the mail queue whenever the Internet link is established.
<p> Put the following command into your PPP or SLIP dialup scripts: </p>
<pre>
/usr/sbin/sendmail -q (whenever the Internet link is up)
</pre>
<p> The exact location of the Postfix sendmail command is system-specific.
Use the command "<b>postconf sendmail_path</b>" to find out where the
Postfix sendmail command is located on your machine. </p>
<p> In order to find out if the mail queue is flushed, use something
like: </p>
<pre>
#!/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
</pre>
<p> If you have disabled <a href="#spontaneous_smtp">spontaneous
SMTP mail delivery</a>, you also need to run the "<b>sendmail -q</b>"
command every now and then while the dialup link is up, so that
newly-posted mail is flushed from the queue. </p>
</ul>
<h2><a name="fantasy">Postfix on hosts without a real Internet
hostname</a></h2>
<p> This section is for hosts that don't have their own 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 addresses with
non-existent domain names. </p>
<p> Note: the following information is Postfix version dependent.
To find out what Postfix version you have, execute the command
"<b>postconf mail_version</b>". </p>
<h3>Solution 1: Postfix version 2.2 and later </h3>
<p> Postfix 2.2 uses the generic(5) address mapping to replace
local fantasy email addresses by valid Internet addresses. This
mapping happens ONLY when mail leaves the machine; not when you
send mail between users on the same machine. </p>
<p> The following example presents additional configuration. You
need to combine this with basic configuration information as
discussed the first half of this document. </p>
<blockquote>
<pre>
1 /etc/postfix/main.cf:
2 smtp_generic_maps = hash:/etc/postfix/generic
3
4 /etc/postfix/generic:
5 his@localdomain.local hisaccount@hisisp.example
6 her@localdomain.local heraccount@herisp.example
7 @localdomain.local hisaccount+local@hisisp.example
</pre>
</blockquote>
<p> When mail is sent to a remote host via SMTP: </p>
<ul>
<li> <p> Line 5 replaces <i>his@localdomain.local</i> by his ISP
mail address, </p>
<li> <p> Line 6 replaces <i>her@localdomain.local</i> by her ISP
mail address, and </p>
<li> <p> Line 7 replaces other local addresses by his ISP account,
with an address extension of +<i>local</i> (this example assumes
that the ISP supports "+" style address extensions). </p>
</ul>
<p>Specify <b>dbm</b> instead of <b>hash</b> if your system uses
<b>dbm</b> files instead of <b>db</b> files. To find out what lookup
tables Postfix supports, use the command "<b>postconf -m</b>". </p>
<p> Execute the command "<b>postmap /etc/postfix/generic</b>"
whenever you change the generic table. </p>
<h3>Solution 2: Postfix version 2.1 and earlier </h3>
<p> The solution with older Postfix systems 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.</p>
<p> The following example presents additional configuration. You
need to combine this with basic configuration information as
discussed the first half of this document. </p>
<blockquote>
<pre>
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
</pre>
</blockquote>
<p> Translation: </p>
<ul>
<li> <p> 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. </p>
<li> <p> Lines 5, 9, 10: This provides the mapping from
"your-login-name@hostname.localdomain" to "your-account@your-isp.com".
This part is required. </p>
<li> <p> 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.
</ul>
<p>Specify <b>dbm</b> instead of <b>hash</b> if your system uses
<b>dbm</b> files instead of <b>db</b> files. To find out what lookup
tables Postfix supports, use the command "<b>postconf -m</b>". </p>
<p> Execute the command "<b>postmap /etc/postfix/canonical</b>"
whenever you change the canonical table. </p>
<p> Execute the command "<b>postmap /etc/postfix/virtual</b>"
whenever you change the virtual table. </p>
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