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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Address Verification Howto</h1>
<hr>
<h2>WARNING WARNING WARNING </h2>
<p> The sender/recipient address verification feature described in this
document is suitable only for low-traffic sites. It performs poorly
under high load and may cause your site to be blacklisted by some
providers. See the "<a href="#limitations">Limitations</a>" section
below for details. </p>
<h2><a name="summary">What Postfix address verification can do for you</a></h2>
<p> Address verification is a feature that allows the Postfix SMTP
server to block a sender (MAIL FROM) or recipient (RCPT TO) address
until the address has been verified to be deliverable. </p>
<p> The technique has obvious uses in order to reject junk mail
with an unreplyable sender address. </p>
<p> The technique may also be useful to block mail for undeliverable
recipients, for example on a mail relay host that does not have a
list of all the valid recipient addresses. This prevents undeliverable
junk mail from entering the queue, so that Postfix doesn't have to
waste resources trying to send MAILER-DAEMON messages back. </p>
<p> This feature is available in Postfix version 2.1 and later. </p>
<p> Topics covered in this document: </p>
<ul>
<li><a href="#how"> How address verification works</a>
<li><a href="#limitations">Limitations of address verification</a>
<li><a href="#recipient">Recipient address verification</a>
<li><a href="#forged_sender">Sender address verification for mail
from frequently forged domains</a>
<li><a href="#sender_always">Sender address verification for all
email</a>
<li><a href="#caching">Address verification database</a>
<li><a href="#dirty_secret">Managing the address verification
database</a>
<li><a href="#probe_routing">Controlling the routing of address
verification probes</a>
<li><a href="#forced_examples">Forced probe routing examples</a>
<li><a href="#forced_limitations">Limitations of forced probe routing</a>
</ul>
<h2><a name="how">How address verification works</a></h2>
<p> A sender or recipient address is verified by probing the nearest
MTA for that address, without actually delivering mail. The nearest
MTA could be Postfix itself, or it could be a remote MTA (SMTP
interruptus). Probe messages are like normal mail, except that
they are never delivered, deferred or bounced; probe messages are
always discarded. </p>
<blockquote>
<table>
<tr>
<td bgcolor="#f0f0ff" align="center" valign="middle"> Internet
</td>
<td align="center" valign="middle"> <tt> -> </tt> </td>
<td bgcolor="#f0f0ff" align="center" valign="middle"> <a
href="smtpd.8.html">Postfix<br> SMTP<br> server</a> </td>
<td align="center" valign="middle"> <tt> <-> </tt> </td>
<td bgcolor="#f0f0ff" align="center" valign="middle"> <a
href="verify.8.html">Postfix<br> verify<br> server</a>
</td>
<td align="center" valign="middle"> <tt> <-> </tt> </td>
<td bgcolor="#f0f0ff" align="center" valign="middle">
Address<br> verification<br> database </td>
</tr>
<tr>
<td> </td>
<td> </td>
<td align="center" valign="middle"> <tt> |</tt><br> probe<br>
messages<br> <tt> v </tt> </td>
<td> </td>
<td align="center" valign="middle"> ^<br> delivery<br>
status<br> <tt> | </tt> </td>
<td> </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td> </td>
<td bgcolor="#f0f0ff" align="center" valign="middle"> Postfix<br>
queue </td>
<td align="center" valign="middle"> <tt> -> </tt> </td>
<td bgcolor="#f0f0ff" align="center" valign="middle"> Postfix<br>
delivery<br> agents </td>
<td> </td>
<td> </td>
</table>
</blockquote>
<p> With Postfix address verification turned on, normal mail will
suffer only a short delay of up to 6 seconds while an address is
being verified for the first time. Once an address status is known,
the status is cached and Postfix replies immediately. </p>
<p> When verification takes too long the Postfix SMTP server defers
the sender or recipient address with a 450 reply. Normal mail
clients will connect again after some delay. The address verification
delay is configurable with the main.cf address_verify_poll_count
and address_verify_poll_delay parameters. See postconf(5) for
details. </p>
<h2><a name="limitations">Limitations of address verification</a></h2>
<ul>
<li> <p> Postfix probes the nearest MTA for the address that is
being verified, without actually sending mail to that address. If
the nearest MTA accepts the address, then Postfix assumes that the
address is deliverable, even when the address will bounce AFTER
that MTA accepts it. </p>
<li> <p> Sites like AOL may blacklist you when you are probing them
too often (a probe is an SMTP session that does not deliver mail),
or when you are probing them too often for a non-existent address.
</p>
<li> <p> Normally, address verification probe messages follow the
same path as regular mail. However, some sites send mail to the
Internet via an intermediate relayhost; this breaks address
verification. See below, section <a href="#probe_routing">"Controlling
the routing of address verification probes"</a>, for how to override
mail routing and for possible limitations when you have to do this.
</p>
<li> <p> Postfix assumes that an address is undeliverable when the
nearest MTA for the address rejects the probe, regardless of the
reason for rejection (client rejected, HELO rejected, MAIL FROM
rejected, etc.). Thus, Postfix rejects mail when the sender's MTA
rejects mail from your machine. This is a good thing. </p>
<li> <p> Unfortunately, some major sites such as YAHOO do not reject
unknown addresses in reply to the RCPT TO command, but report a
delivery failure in response to end of DATA after a message is
transferred. Postfix address verification does not work with such
sites. </p>
<li> <p> By default, Postfix probe messages have "postmaster@$myorigin"
as the sender address. This is SAFE because the Postfix SMTP server
does not reject mail for this address. </p>
<p> You can change this into the null address ("address_verify_sender
="). This is UNSAFE because address probes will fail with
mis-configured sites that reject MAIL FROM: <>, while
probes from "postmaster@$myorigin" would succeed. </p>
</ul>
<h2><a name="recipient">Recipient address verification</a></h2>
<p> As mentioned earlier, recipient address verification may be
useful to block mail for undeliverable recipients on a mail relay
host that does not have a list of all valid recipient addresses.
This can help to prevent the mail queue from filling up with
MAILER-DAEMON messages. </p>
<p> Recipient address verification is relatively straightforward
and there are no surprises. If a recipient probe fails, then Postfix
rejects mail for the recipient address. If a recipient probe
succeeds, then Postfix accepts mail for the recipient address. </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
permit_mynetworks
reject_unauth_destination
...
reject_unknown_recipient_domain
reject_unverified_recipient
...
</pre>
</blockquote>
<p> The "reject_unknown_recipient_domain" restriction blocks mail
for non-existent domains. Putting this before "reject_unverified_recipient"
avoids the overhead of generating unnecessary probe messages. </p>
<p> The unverified_recipient_reject_code parameter (default 450)
specifies how Postfix replies when a recipient address is known to
bounce. Change this setting into 550 when you trust Postfix's
judgments. </p>
<h2><a name="forged_sender">Sender address verification for mail from frequently forged domains</a></h2>
<p> It is relatively safe to turn on sender address verification for
specific domains that often appear in forged email. </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sender_restrictions = hash:/etc/postfix/sender_access
unverified_sender_reject_code = 550
# Note 1: Be sure to read the "<a href="#caching">Caching</a>" section below!
# Note 2: Avoid hash files here. Use btree instead.
address_verify_map = btree:/var/mta/verify
/etc/postfix/sender_access:
aol.com reject_unverified_sender
hotmail.com reject_unverified_sender
bigfoot.com reject_unverified_sender
... etcetera ...
</pre>
</blockquote>
<p> A list of frequently forged MAIL FROM domains can be found at
http://www.monkeys.com/anti-spam/filtering/sender-domain-validate.in.
</p>
<p> NOTE: One of the first things you might want to do is to turn
on sender address verification for all your own domains. </p>
<h2><a name="sender_always">Sender address verification for all
email</a></h2>
<p> Unfortunately, sender address verification cannot simply be
turned on for all email - you are likely to lose legitimate mail
from mis-configured systems. You almost certainly will have to set
up white lists for specific addresses, or even for entire domains.
</p>
<p> To find out how sender address verification would affect your
mail, specify "warn_if_reject reject_unverified_sender" so that
you can see what mail would be blocked: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
smtpd_sender_restrictions =
permit_mynetworks
...
check_sender_access hash:/etc/postfix/sender_access
reject_unknown_sender_domain
warn_if_reject reject_unverified_sender
...
# Note 1: Be sure to read the "<a href="#caching">Caching</a>" section below!
# Note 2: Avoid hash files here. Use btree instead.
address_verify_map = btree:/var/mta/verify
</pre>
</blockquote>
<p> This is also a good way to populate your cache with address
verification results before you start to actually reject mail. </p>
<p> The sender_access restriction is needed to whitelist domains
or addresses that are known to be OK. Although Postfix will not
mark a known-to-be-good address as bad after a probe fails, it is
better to be safe than sorry. </p>
<p> NOTE: You will have to whitelist sites such as securityfocus.com
and other sites that operate mailing lists that use a different
sender address for each posting (VERP). Such addresses pollute
the address verification cache quickly, and generate unnecessary
sender verification probes. </p>
<blockquote>
<pre>
/etc/postfix/sender_access
securityfocus.com OK
...
</pre>
</blockquote>
<p> The "reject_unknown_sender_domain" restriction blocks mail from
non-existent domains. Putting this before "reject_unverified_sender"
avoids the overhead of generating unnecessary probe messages. </p>
<p> The unverified_sender_reject_code parameter (default 450)
specifies how Postfix replies when a sender address is known to
bounce. Change this setting into 550 when you trust Postfix's
judgments. </p>
<h2><a name="caching">Address verification database</a></h2>
<p> NOTE: By default, address verification information is not stored
in a persistent file. You have to specify one in main.cf (see
below). Persistent storage is off by default because it may need
more disk space than is available in your file system. </p>
<p> Address verification information is cached by the Postfix verify
daemon. Postfix has a bunch of parameters that control the caching
of positive and negative results. Refer to the verify(8) manual
page for details. </p>
<p> The address_verify_map (NOTE: singular) configuration parameter
specifies an optional persistent database for sender address
verification results. If you don't specify a file, all address
verification information is lost after "postfix reload" or "postfix
stop". </p>
<p> If your /var file system has sufficient space, try: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
# Note: avoid hash files here. Use btree instead.
address_verify_map = btree:/var/mta/verify
</pre>
</blockquote>
<p> NOTE: Do not put this file in a file system that may run out
of space. When the address verification table gets corrupted the
world comes to an end and YOU will have to MANUALLY fix things as
described in the next section. Meanwhile, you will not receive mail
via SMTP. </p>
<p> The verify(8) daemon process will create a new database when
none exists, and will open/create the file before it enters the
chroot jail and before it drops root privileges. </p>
<h2><a name="dirty_secret">Managing the address verification
database</a></h2>
<p> The verify(8) manual page describes parameters that control
how long information remains cached before it needs to be refreshed,
and how long information can remain "unrefreshed" before it expires.
Postfix uses different controls for positive results (address was
accepted) and for negative results (address was rejected). </p>
<p> Right now, no tools are provided to manage the address verification
database. If the file gets too big, or if it gets corrupted, you
can manually rename or delete the file and run "postfix reload".
The new verify daemon process will then create a new database. </p>
<h2><a name="probe_routing">Controlling the routing of address
verification probes</a></h2>
<p> By default, Postfix sends address verification probe messages
via the same route as regular mail, because that normally produces
the most accurate result. It's no good to verify a local address
by connecting to your own SMTP port; that just triggers all kinds
of mailer loop alarms. The same is true for any destination that
your machine is best MX host for: hidden domains, virtual domains,
etc. </p>
<p> However, some sites have a complex infrastructure where mail
is not sent directly to the Internet, but is instead given to an
intermediate relayhost. This is a problem for address verification,
because remote Internet addresses can be verified only when Postfix
can access remote destinations directly. </p>
<p> For this reason, Postfix allows you to override the routing
parameters when it delivers an address verification probe message.
</p>
<p> First, the address_verify_relayhost parameter allows you to
override the relayhost setting, and the address_verify_transport_maps
parameter allows you to override the transport_maps setting. </p>
<p> Second, each address class is given its own address verification
version of the message delivery transport, as shown in the table
below. Address classes are defined in the ADDRESS_CLASS_README
file. </p>
<blockquote>
<table border="1">
<tr> <th> Domain list </th> <th> Regular transport</th> <th> Verify
transport </th> </tr>
<tr> <td> mydestination </td> <td> local_transport </td> <td>
address_verify_local_transport </td> </tr>
<tr> <td> virtual_alias_domains </td> <td> (not applicable) </td>
<td> (not applicable) </td> </tr>
<tr> <td> virtual_mailbox_domains </td> <td> virtual_transport
</td> <td> address_verify_virtual_transport </td> </tr>
<tr> <td> relay_domains </td> <td> relay_transport </td> <td>
address_verify_relay_transport </td> </tr>
<tr> <td> (not applicable) </td> <td> default_transport </td> <td>
address_verify_default_transport </td> </tr>
</table>
</blockquote>
<p> By default, the parameters that control delivery of address
probes have the same value as the parameters that control normal
mail delivery. </p>
<h2><a name="forced_examples">Forced probe routing examples</a></h2>
<p> In a typical scenario one would override the relayhost setting
for address verification probes and leave everything else alone:
</p>
<blockquote>
<pre>
/etc/postfix/main.cf:
relayhost = $mydomain
address_verify_relayhost =
...
</pre>
</blockquote>
<p> Sites behind a network address translation box might have to
use a different SMTP client that sends the correct hostname
information: </p>
<blockquote>
<pre>
/etc/postfix/main.cf:
relayhost = $mydomain
address_verify_relayhost =
address_verify_default_transport = direct_smtp
/etc/postfix/master.cf:
direct_smtp .. .. .. .. .. .. .. .. .. smtp
-o smtp_helo_name=nat.box.tld
</pre>
</blockquote>
<h2><a name="forced_limitations">Limitations of forced probe routing</a></h2>
<p> Inconsistencies can happen when probe messages don't follow
the same path as regular mail. For example, a message can be
accepted when it follows the regular route while an otherwise
identical probe message is rejected when it follows the forced
route. The opposite can happen, too, but is less likely. </p>
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