diff --git a/howto/enable.dnsbl.md b/howto/enable.dnsbl.md
new file mode 100644
index 00000000..9d2b4097
--- /dev/null
+++ b/howto/enable.dnsbl.md
@@ -0,0 +1,26 @@
+# Enable DNSBL service in Postfix to reduce spam
+
+You can enable additional DNSBL services in Postfix to reduce spam. We use
+`zen.spamhaus.org` for example below.
+
+* Open Postfix config file `/etc/postfix/main.cf` or
+`/usr/local/etc/postfix/main.cf` (on FreeBSD), append
+`reject_rbl_client zen.spamhaus.org` to parameter `smtpd_recipient_restrictions`.
+Final setting looks like below:
+
+```
+smtpd_recipient_restrictions = ..., reject_unauth_destination, reject_rbl_client zen.spamhaus.org
+```
+
+It must be placed after `reject_unauth_destination`. You can add more DNSBL
+services after `reject_unauth_destination`, and they will be queried in the
+specified order.
+
+* Restart or reload Postfix service is required.
+
+## References
+
+* [Postfix Configuration Parameters: reject_rbl_client](http://www.iredmail.org/postconf.5.html#reject_rbl_client)
+* [Spamhaus website](http://www.spamhaus.org)
+
+    * [Spamhaus DNSBL Usage Terms](https://www.spamhaus.org/organization/dnsblusage/)
diff --git a/html/enable.dnsbl.html b/html/enable.dnsbl.html
new file mode 100644
index 00000000..a57b888d
--- /dev/null
+++ b/html/enable.dnsbl.html
@@ -0,0 +1,49 @@
+<html>
+    <head>
+        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
+        <title>Enable DNSBL service in Postfix to reduce spam</title>
+        <link href="./css/markdown.css" rel="stylesheet"></head>
+    </head>
+    <body>
+    
+    <div id="navigation">
+        <a href="http://www.iredmail.org" target="_blank">iRedMail web site</a>
+    
+        // <a href="./index.html">Document Index</a>
+    </div><h1 id="enable-dnsbl-service-in-postfix-to-reduce-spam">Enable DNSBL service in Postfix to reduce spam</h1>
+<p>You can enable additional DNSBL services in Postfix to reduce spam. We use
+<code>zen.spamhaus.org</code> for example below.</p>
+<ul>
+<li>Open Postfix config file <code>/etc/postfix/main.cf</code> or
+<code>/usr/local/etc/postfix/main.cf</code> (on FreeBSD), append
+<code>reject_rbl_client zen.spamhaus.org</code> to parameter <code>smtpd_recipient_restrictions</code>.
+Final setting looks like below:</li>
+</ul>
+<pre><code>smtpd_recipient_restrictions = ..., reject_unauth_destination, reject_rbl_client zen.spamhaus.org
+</code></pre>
+
+<p>It must be placed after <code>reject_unauth_destination</code>. You can add more DNSBL
+services after <code>reject_unauth_destination</code>, and they will be queried in the
+specified order.</p>
+<ul>
+<li>Restart or reload Postfix service is required.</li>
+</ul>
+<h2 id="references">References</h2>
+<ul>
+<li><a href="http://www.iredmail.org/postconf.5.html#reject_rbl_client">Postfix Configuration Parameters: reject_rbl_client</a></li>
+<li>
+<p><a href="http://www.spamhaus.org">Spamhaus website</a></p>
+<ul>
+<li><a href="https://www.spamhaus.org/organization/dnsblusage/">Spamhaus DNSBL Usage Terms</a></li>
+</ul>
+</li>
+</ul><p style="text-align: center; color: grey;">Document published under a <a href="http://creativecommons.org/licenses/by-nd/3.0/us/" target="_blank">CC BY-ND 3.0</a> license. If you found something wrong, please do <a href="http://www.iredmail.org/contact.html">contact us</a> to fix it.<script>
+  (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+  (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+  m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+  })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
+
+  ga('create', 'UA-3293801-21', 'auto');
+  ga('send', 'pageview');
+</script>
+</body></html>
\ No newline at end of file
diff --git a/html/index.html b/html/index.html
index 6cdcc171..b63fad69 100644
--- a/html/index.html
+++ b/html/index.html
@@ -57,6 +57,7 @@
 <li><a href="amavisd.per-recipient.policy.lookup.html">Amavisd: Enable per-recipient policy lookup</a></li>
 <li><a href="change.hostname.html">Change server hostname</a></li>
 <li><a href="dovecot.master.user.html">Dovecot Master User</a></li>
+<li><a href="enable.dnsbl.html">Enable DNSBL service in Postfix to reduce spam</a></li>
 <li><a href="force.user.to.change.password.html">Force mail user to change password in 90 days</a></li>
 <li><a href="ignore.trash.folder.in.quota.html">Ignore Trash folder in mailbox quota</a></li>
 <li><a href="ldap.add.alias.domain.html">LDAP: Add an alias domain</a></li>
diff --git a/html/upgrade.iredmail.0.9.0-0.9.1.html b/html/upgrade.iredmail.0.9.0-0.9.1.html
index 81b8fb1d..844eb388 100644
--- a/html/upgrade.iredmail.0.9.0-0.9.1.html
+++ b/html/upgrade.iredmail.0.9.0-0.9.1.html
@@ -23,7 +23,6 @@
 <li><a href="#fixed-incorrect-path-of-command-sogo-tool-on-openbsd">Fixed: Incorrect path of command sogo-tool on OpenBSD</a></li>
 <li><a href="#optional-setup-fail2ban-to-monitor-password-failures-in-sogo-log-file">[OPTIONAL] Setup Fail2ban to monitor password failures in SOGo log file</a></li>
 <li><a href="#optional-add-two-more-fail2ban-filter-regular-expressios-to-help-catch-spam">[OPTIONAL] Add two more Fail2ban filter regular expressios to help catch spam</a></li>
-<li><a href="#optional-enable-dnsbl-service-zenspamhausorg-in-postfix-to-reduce-spam">[OPTIONAL] Enable DNSBL service zen.spamhaus.org in Postfix to reduce spam</a></li>
 </ul>
 </li>
 <li><a href="#openldap-backend-special">OpenLDAP backend special</a><ul>
@@ -247,22 +246,6 @@ ignoreregex =
 </code></pre>
 
 <p>Restarting Fail2ban service is required.</p>
-<h3 id="optional-enable-dnsbl-service-zenspamhausorg-in-postfix-to-reduce-spam">[OPTIONAL] Enable DNSBL service zen.spamhaus.org in Postfix to reduce spam</h3>
-<p>Note: this is optional. It's enabled by default since iRedMail-0.9.1.</p>
-<ul>
-<li>Open Postfix config file <code>/etc/postfix/main.cf</code> or
-<code>/usr/local/etc/postfix/main.cf</code> (on FreeBSD), append
-<code>reject_rbl_client zen.spamhaus.org</code> to parameter <code>smtpd_recipient_restrictions</code>.
-Final setting looks like below:</li>
-</ul>
-<pre><code>smtpd_recipient_restrictions = ..., reject_unauth_destination, reject_rbl_client zen.spamhaus.org
-</code></pre>
-
-<ul>
-<li>Restart or reload Postfix service.</li>
-</ul>
-<p>You can read <a href="http://www.spamhaus.org">spamhaus.org</a> for more details about its
-RBL service.</p>
 <h2 id="openldap-backend-special">OpenLDAP backend special</h2>
 <h3 id="fixed-not-backup-sogo-database">Fixed: not backup SOGo database</h3>
 <p>Note: this step is not applicable if you don't use SOGo groupware.</p>
diff --git a/upgrade/0-upgrade.iredmail.0.9.0-0.9.1.md b/upgrade/0-upgrade.iredmail.0.9.0-0.9.1.md
index 62971199..12703eb8 100644
--- a/upgrade/0-upgrade.iredmail.0.9.0-0.9.1.md
+++ b/upgrade/0-upgrade.iredmail.0.9.0-0.9.1.md
@@ -235,24 +235,6 @@ ignoreregex =
 
 Restarting Fail2ban service is required.
 
-### [OPTIONAL] Enable DNSBL service zen.spamhaus.org in Postfix to reduce spam
-
-Note: this is optional. It's enabled by default since iRedMail-0.9.1.
-
-* Open Postfix config file `/etc/postfix/main.cf` or
-`/usr/local/etc/postfix/main.cf` (on FreeBSD), append
-`reject_rbl_client zen.spamhaus.org` to parameter `smtpd_recipient_restrictions`.
-Final setting looks like below:
-
-```
-smtpd_recipient_restrictions = ..., reject_unauth_destination, reject_rbl_client zen.spamhaus.org
-```
-
-* Restart or reload Postfix service.
-
-You can read [spamhaus.org](http://www.spamhaus.org) for more details about its
-RBL service.
-
 ## OpenLDAP backend special
 
 ### Fixed: not backup SOGo database
