659 lines
24 KiB
HTML
659 lines
24 KiB
HTML
<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN"
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"https://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<title>Postfix Address Verification </title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link rel='stylesheet' type='text/css' href='postfix-doc.css'>
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</head>
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<body>
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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Address Verification Howto</h1>
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<hr>
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<h2>WARNING </h2>
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<p> Recipient address verification may cause an increased load on
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down-stream servers in the case of a dictionary attack or a flood
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of backscatter bounces. Sender address verification may cause your
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site to be denylisted by some providers. See also the "<a
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href="#limitations">Limitations</a>" section below for more. </p>
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<h2><a name="summary">What Postfix address verification can do for you</a></h2>
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<p> Address verification is a feature that allows the Postfix SMTP
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server to block a sender (MAIL FROM) or recipient (RCPT TO) address
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until the address has been verified to be deliverable. </p>
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<p> The technique has obvious uses to reject junk mail
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with an unreplyable sender address. </p>
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<p> The technique is also useful to block mail for undeliverable
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recipients, for example on a mail relay host that does not have a
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list of all the valid recipient addresses. This prevents undeliverable
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junk mail from entering the queue, so that Postfix doesn't have to
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waste resources trying to send MAILER-DAEMON messages back. </p>
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<p> This feature is available in Postfix version 2.1 and later. </p>
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<p> Topics covered in this document: </p>
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<ul>
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<li><a href="#how"> How address verification works</a>
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<li><a href="#limitations">Limitations of address verification</a>
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<li><a href="#recipient">Recipient address verification</a>
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<li><a href="#forged_sender">Sender address verification for mail
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from frequently forged domains</a>
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<li><a href="#sender_always">Sender address verification for all
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email</a>
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<li><a href="#caching">Address verification database</a>
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<li><a href="#dirty_secret">Managing the address verification
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database</a>
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<li><a href="#probe_routing">Controlling the routing of address
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verification probes</a>
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<li><a href="#forced_examples">Forced probe routing examples</a>
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<li><a href="#forced_limitations">Limitations of forced probe routing</a>
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</ul>
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<h2><a name="how">How address verification works</a></h2>
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<p> A Postfix MTA verifies a sender or recipient address by probing
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the preferred MTAs
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for that address, without actually delivering mail. The preferred
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MTAs could include the Postfix MTA itself, or some remote MTAs
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(SMTP
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interruptus). Probe messages are like normal mail, except that
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they are never delivered, deferred or bounced; probe messages are
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always discarded. </p>
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<blockquote>
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<table border="0">
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<tr>
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<td rowspan="2" colspan="5" align="center" valign="middle">
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</td>
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<td rowspan="3" align="center" valign="bottom"> <tt> -> </tt>
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</td>
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<td rowspan="3" align="center" valign="middle"> probe<br>
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message </td>
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<td rowspan="3" align="center" valign="middle"> <tt> -> </tt>
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</td>
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<td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle">
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Postfix<br> mail<br> queue </td>
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</tr>
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<tr> <td> </td> </tr>
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<tr>
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<td rowspan="3" align="center" valign="middle"> Internet </td>
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<td rowspan="3" align="center" valign="middle"> <tt> -> </tt>
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</td>
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<td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle">
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<a href="smtpd.8.html">Postfix<br> SMTP<br> server</a> </td>
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<td rowspan="3" align="center" valign="middle"> <tt> <->
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</tt> </td>
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<td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle">
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<a href="verify.8.html">Postfix<br> verify<br> server</a>
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</td>
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</tr>
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<tr>
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<td rowspan="1" colspan="3"> </td>
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<td rowspan="1" align="center" valign="middle"> <tt> |</tt><br>
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<tt> v</tt> </td>
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</tr>
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<tr>
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<td rowspan="3" align="center" valign="top"> <tt> <- </tt>
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</td>
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<td rowspan="3" align="center" valign="middle"> probe<br>
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status </td>
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<td rowspan="3" align="center" valign="middle"> <tt> <- </tt>
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</td>
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<td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle">
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Postfix<br> delivery<br> agents </td>
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<td rowspan="3" align="left" valign="middle"> <tt>-></tt>
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Local<br> <tt>-></tt> Remote</td>
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</tr>
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<tr>
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<td rowspan="3" colspan="4" align="center" valign="middle">
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</td>
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<td rowspan="3" align="center" valign="middle"> <tt>
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^</tt><br> <tt> |</tt><br> <tt> v</tt> </td>
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</tr>
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<tr> <td> </td> </tr>
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<tr> <td colspan="4"> </td> </tr>
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<tr>
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<td colspan="4" align="center" valign="middle"> </td>
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<td bgcolor="#f0f0ff" align="center" valign="middle">
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Address<br> verification<br> database </td>
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</tr>
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</table>
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</blockquote>
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<p> With Postfix address verification turned on, normal mail will
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suffer only a short delay of up to 6 seconds while an address is
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being verified for the first time. Once an address status is known,
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the status is cached and Postfix replies immediately. </p>
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<p> When verification takes too long the Postfix SMTP server defers
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the sender or recipient address with a 450 reply. Normal mail
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clients will connect again after some delay. The address verification
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delay is configurable with the main.cf address_verify_poll_count
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and address_verify_poll_delay parameters. See postconf(5) for
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details. </p>
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<h2><a name="limitations">Limitations of address verification</a></h2>
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<ul>
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<li> <p> Postfix assumes that a remote SMTP server will reject
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unknown addresses in reply to the RCPT TO command. However, some
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sites report this in reply to the DATA command. For such sites
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you may configure a workaround with the smtp_address_verify_target
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parameter (Postfix 3.0 and later). </p>
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<li> <p> When verifying a remote address, Postfix probes the preferred
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MTAs for that address, without actually delivering mail. If
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a preferred MTA accepts the address, then Postfix assumes that the
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address is deliverable. In reality, mail for a remote address can
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bounce AFTER a preferred MTA accepts the recipient address, or AFTER
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a preferred MTA accepts the message content. </p>
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<li> <p> Some sites may denylist you when you are probing them
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too often (a probe is an SMTP session that does not deliver mail),
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or when you are probing them too often for a non-existent address.
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This is one reason why you should use sender address verification
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sparingly, if at all, when your site receives lots of email. </p>
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<li> <p> Normally, address verification probe messages follow the
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same path as regular mail. However, some sites send mail to the
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Internet via an intermediate relayhost; this breaks address
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verification. See below, section <a href="#probe_routing">"Controlling
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the routing of address verification probes"</a>, for how to override
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mail routing and for possible limitations when you have to do this.
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</p>
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<li> <p> Postfix assumes that an address is undeliverable when a
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preferred MTA for the address rejects the probe, regardless of the
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reason for rejection (client rejected, HELO rejected, MAIL FROM
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rejected, etc.). Thus, Postfix rejects an address when a preferred
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MTA for that address rejects mail from your machine for any reason.
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This is not a limitation, but it is mentioned here just in case
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people believe that it is a limitation. </p>
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<li> <p> Unfortunately, some sites do not reject unknown addresses
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in reply to the RCPT TO or DATA command, but instead report a
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delivery failure in response to end of DATA after a message is
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transferred. Postfix address verification does not work with such
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sites. </p>
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<li> <p> By default, Postfix probe messages have a sender address
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"double-bounce@$myorigin" (with Postfix versions before 2.5, the
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default
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is "postmaster@$myorigin"). This is SAFE because the Postfix SMTP
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server does not reject mail for this address. </p>
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<p> You can change the probe sender address into the null address
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("address_verify_sender
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="). This is UNSAFE because address probes will fail with
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mis-configured sites that reject MAIL FROM: <>, while
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probes from "double-bounce@$myorigin" would succeed. </p>
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<li> <p> The downside of using a non-empty sender address is that
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the address may end up on spammer mailing lists. Although Postfix
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always discards mail to the double-bounce address, this still results
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in wasted network bandwidth and server capacity. To defeat
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address harvesting, Postfix 2.9 and later support time-dependent
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sender addresses when you specify a non-zero address_verify_sender_ttl
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value. </p>
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</ul>
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<h2><a name="recipient">Recipient address verification</a></h2>
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<p> As mentioned earlier, recipient address verification is
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useful to block mail for undeliverable recipients on a mail relay
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host that does not have a list of all valid recipient addresses.
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This can help to prevent the mail queue from filling up with
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MAILER-DAEMON messages. </p>
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<p> Recipient address verification is relatively straightforward
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and there are no surprises. If a recipient probe fails, then Postfix
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rejects mail for the recipient address. If a recipient probe
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succeeds, then Postfix accepts mail for the recipient address.
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However, recipient address verification probes can increase the
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load on down-stream MTAs when you're being flooded by backscatter
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bounces, or when some spammer is mounting a dictionary attack. </p>
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<p> By default, address verification results are saved in a <a
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href="#caching">persistent database</a> (Postfix version 2.7 and
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later; with earlier versions, specify the database in main.cf as
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described later). The persistent database helps to avoid probing
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the same address repeatedly. </p>
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<blockquote>
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<pre>
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/etc/postfix/main.cf:
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smtpd_recipient_restrictions =
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permit_mynetworks
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# reject_unauth_destination is not needed here if the mail
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# relay policy is specified under smtpd_relay_restrictions
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# (available with Postfix 2.10 and later).
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reject_unauth_destination
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...
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reject_unknown_recipient_domain
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reject_unverified_recipient
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...
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# Postfix 2.6 and later privacy feature.
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# unverified_recipient_reject_reason = Address lookup failed
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# Postfix 3.2 and earlier workaround.
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# Do not set enable_original_recipient=no. This prevents Postfix
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# from saving the recipient address verification result under
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# the original address, when the address verification probe
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# message goes through address aliasing or canonical mapping.
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</pre>
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</blockquote>
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<p> The "reject_unknown_recipient_domain" restriction blocks mail
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for non-existent domains. Putting this before "reject_unverified_recipient"
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avoids the overhead of generating unnecessary probe messages. </p>
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<p> The unverified_recipient_reject_code parameter (default 450)
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specifies the numerical Postfix SMTP server reply code when a
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recipient address is known to
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bounce. Change this setting into 550 when you trust Postfix's
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judgments. </p>
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<p> The following features are available in Postfix 2.6 and later.
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</p>
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<p> The unverified_recipient_defer_code parameter (default 450)
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specifies the numerical Postfix SMTP server reply code when a
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recipient address probe fails with some temporary error. Some sites
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insist on changing this into 250. NOTE: This change turns MX servers
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into backscatter sources when the load is high. </p>
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<p> The unverified_recipient_reject_reason parameter (default:
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empty) specifies fixed text that Postfix will send to remote SMTP
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clients, instead of sending actual address verification details.
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Do not specify the SMTP status code or enhanced status code. </p>
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<p> The unverified_recipient_tempfail_action parameter (default:
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defer_if_permit) specifies the Postfix SMTP server action when a
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recipient address verification probe fails with some temporary
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error. </p>
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<h2><a name="forged_sender">Sender address verification for mail from frequently forged domains</a></h2>
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<p> Only for very small sites, it is relatively safe to turn on
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sender address verification for specific domains that often appear
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in forged email. </p>
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<blockquote>
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<pre>
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/etc/postfix/main.cf:
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smtpd_sender_restrictions = hash:/etc/postfix/sender_access
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unverified_sender_reject_code = 550
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# Postfix 2.6 and later.
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# unverified_sender_defer_code = 250
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# Default setting for Postfix 2.7 and later.
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# Note 1: Be sure to read the "<a href="#caching">Caching</a>" section below!
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# Note 2: Avoid hash files here. Use btree or lmdb instead.
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address_verify_map = btree:/var/lib/postfix/verify
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# Postfix 3.2 and earlier workaround.
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# Do not set enable_original_recipient=no. This prevents Postfix
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# from saving the sender address verification result under the
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# original address, when the address verification probe message
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# goes through address aliasing or canonical mapping.
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/etc/postfix/sender_access:
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# Don't do this when you handle lots of email.
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aol.com reject_unverified_sender
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hotmail.com reject_unverified_sender
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bigfoot.com reject_unverified_sender
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... etcetera ...
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</pre>
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</blockquote>
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<p> At some point in cyberspace/time, a list of frequently forged
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MAIL FROM domains was archived at
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https://web.archive.org/web/20080526153208/http://www.monkeys.com/anti-spam/filtering/sender-domain-validate.in. </p>
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<p> NOTE: One of the first things you might want to do is to turn
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on sender address verification for all your own domains. </p>
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<h2><a name="sender_always">Sender address verification for all
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email</a></h2>
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<p> Unfortunately, sender address verification cannot simply be
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turned on for all email - you are likely to lose legitimate mail
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from mis-configured systems. You almost certainly will have to set
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up allow lists for specific addresses, or even for entire domains.
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</p>
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<p> To find out how sender address verification would affect your
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mail, specify "warn_if_reject reject_unverified_sender" so that
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you can see what mail would be blocked: </p>
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<blockquote>
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<pre>
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/etc/postfix/main.cf:
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smtpd_sender_restrictions =
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permit_mynetworks
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...
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check_sender_access hash:/etc/postfix/sender_access
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reject_unknown_sender_domain
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warn_if_reject reject_unverified_sender
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...
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# Postfix 2.6 and later.
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# unverified_sender_reject_reason = Address verification failed
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# Default setting for Postfix 2.7 and later.
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# Note 1: Be sure to read the "<a href="#caching">Caching</a>" section below!
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# Note 2: Avoid hash files here. Use btree or lmdb instead.
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address_verify_map = btree:/var/lib/postfix/verify
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</pre>
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</blockquote>
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<p> This is also a good way to populate your cache with address
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verification results before you start to actually reject mail. </p>
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<p> The sender_access restriction is needed to allowlist domains
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or addresses that are known to be OK. Although Postfix will not
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mark a known-to-be-good address as bad after a probe fails, it is
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better to be safe than sorry. </p>
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<p> NOTE: You will have to allowlist sites such as securityfocus.com
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and other sites that operate mailing lists that use a different
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sender address for each posting (VERP). Such addresses pollute
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the address verification cache quickly, and generate unnecessary
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sender verification probes. </p>
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<blockquote>
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<pre>
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/etc/postfix/sender_access
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securityfocus.com OK
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...
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</pre>
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</blockquote>
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<p> The "reject_unknown_sender_domain" restriction blocks mail from
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non-existent domains. Putting this before "reject_unverified_sender"
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avoids the overhead of generating unnecessary probe messages. </p>
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<p> The unverified_sender_reject_code parameter (default 450)
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specifies the numerical Postfix server reply code when a sender
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address is known to
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bounce. Change this setting into 550 when you trust Postfix's
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judgments. </p>
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<p> The following features are available in Postfix 2.6 and later.
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</p>
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<p> The unverified_sender_defer_code parameter (default 450) specifies
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the numerical Postfix SMTP server reply code when a sender address
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verification probe fails with some temporary error. Specify a valid
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2xx or 4xx code. </p>
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<p> The unverified_sender_reject_reason parameter (default:
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empty) specifies fixed text that Postfix will send to remote SMTP
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clients, instead of sending actual address verification details.
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Do not specify the SMTP status code or enhanced status code. </p>
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<p> The unverified_sender_tempfail_action parameter (default:
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defer_if_permit) specifies the Postfix SMTP server action when a
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sender address verification probe fails with some temporary error.
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</p>
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<h2><a name="caching">Address verification database</a></h2>
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<p> To improve performance, the Postfix verify(8) daemon can save
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address verification results to a persistent database. This is
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enabled by default with Postfix 2.7 and later. The
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address_verify_map (NOTE: singular) configuration parameter specifies
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persistent storage for sender or recipient address verification
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results. If you specify an empty value, all address verification
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results are lost after "postfix reload" or "postfix stop". </p>
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<blockquote>
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<pre>
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# Example 1: Default setting for Postfix 2.7 and later.
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# Note: avoid hash files here. Use btree or lmdb instead.
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/etc/postfix/main.cf:
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address_verify_map = btree:$data_directory/verify_cache
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# Example 2: Shared persistent lmdb: cache (Postfix 2.11 or later).
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# Disable automatic cache cleanup in all Postfix instances except
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# for one instance that will be responsible for cache cleanup.
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/etc/postfix/main.cf:
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address_verify_map = lmdb:$data_directory/verify_cache
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# address_verify_cache_cleanup_interval = 0
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# Example 3: Shared persistent btree: cache (Postfix 2.9 or later).
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# Disable automatic cache cleanup in all Postfix instances except
|
|
# for one instance that will be responsible for cache cleanup.
|
|
/etc/postfix/main.cf:
|
|
address_verify_map = proxy:btree:$data_directory/verify_cache
|
|
# address_verify_cache_cleanup_interval = 0
|
|
|
|
# Example 4: Shared memory cache (requires Postfix 2.9 or later).
|
|
# Disable automatic cache cleanup in all Postfix instances.
|
|
# See memcache_table(5) for details.
|
|
/etc/postfix/main.cf:
|
|
address_verify_map = memcache:/etc/postfix/verify-memcache.cf
|
|
address_verify_cache_cleanup_interval = 0
|
|
|
|
# Example 5: Default setting for Postfix 2.6 and earlier.
|
|
# This uses non-persistent storage only.
|
|
/etc/postfix/main.cf:
|
|
address_verify_map =
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p> NOTE 1: The database file should be stored under a Postfix-owned
|
|
directory, such as $data_directory. </p>
|
|
|
|
<blockquote> As of version 2.5, Postfix no longer uses root privileges
|
|
when opening this file. To maintain backwards compatibility, an
|
|
attempt to open the file under a non-Postfix directory is redirected
|
|
to the Postfix-owned data_directory, and a warning is logged. If
|
|
you wish to continue using a pre-existing database file, change its
|
|
file ownership to the account specified with the mail_owner parameter,
|
|
and either move the file to the data_directory, or move it to some
|
|
other Postfix-owned directory. </blockquote>
|
|
|
|
<p> NOTE 2: 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> NOTE 3: The verify(8) daemon will create a new database when
|
|
none exists. It will open or create the file before entering the
|
|
chroot jail. </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 address verification results are cached before they need to
|
|
be refreshed, and how long results can remain "unrefreshed" before
|
|
they expire. Postfix uses different controls for positive results
|
|
(address was accepted) and for negative results (address was rejected,
|
|
or address verification failed for some other reason). </p>
|
|
|
|
<p> The verify(8) daemon will periodically remove expired entries
|
|
from the address verification database, and log the number of entries
|
|
retained and dropped (Postfix versions 2.7 and later). A cleanup
|
|
run is logged as "partial" when the daemon terminates early because
|
|
of "postfix reload, "postfix stop", or because the daemon received
|
|
no requests for $max_idle seconds. Postfix versions 2.6 and earlier
|
|
do not implement automatic address verification database cleanup.
|
|
There, the database is managed manually as described next. </p>
|
|
|
|
<p> When the address verification database file becomes too big,
|
|
or when it becomes corrupted, the solution is to manually rename
|
|
or delete (NOT: truncate) the file and run "postfix reload". The
|
|
verify(8) daemon will then create a new database file. </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.
|
|
The address_verify_sender_dependent_relayhost_maps parameter
|
|
does the same for sender-dependent relayhost selection. </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>
|
|
|
|
</body>
|
|
|
|
</html>
|