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 4a31a06e..fa028a2d 100644
--- a/html/upgrade.iredmail.0.9.0-0.9.1.html
+++ b/html/upgrade.iredmail.0.9.0-0.9.1.html
@@ -16,6 +16,7 @@
 <li><a href="#upgrade-iredmail-from-090-to-091">Upgrade iRedMail from 0.9.0 to 0.9.1</a><ul>
 <li><a href="#changelog">ChangeLog</a></li>
 <li><a href="#general-all-backends-should-apply-these-steps">General (All backends should apply these steps)</a><ul>
+<li><a href="#optional-fixed-return-receipt-response-rejected-by-iredapd-plugin-reject_null_sender">[OPTIONAL] Fixed: return receipt response rejected by iRedAPD plugin reject_null_sender</a></li>
 <li><a href="#fixed-incorrect-path-of-command-sogo-tool-on-openbsd">Fixed: Incorrect path of command sogo-tool on OpenBSD</a></li>
 </ul>
 </li>
@@ -40,11 +41,39 @@
 <p>WARNING: This is still a working in progress draft document, do <strong>NOT</strong> apply it.</p>
 <h2 id="changelog">ChangeLog</h2>
 <ul>
-<li>2015-02-02: [All backends] Fixed: Not backup SOGo database. Note: this step is not applicable if you don't use SOGo groupware.</li>
+<li>2015-02-04: [All backends] [<strong>OPTIONAL</strong>] Fixed: return receipt response rejected
+              by iRedAPD plugin <code>reject_null_sender</code>.</li>
+<li>2015-02-02: [All backends] Fixed: Not backup SOGo database. Note: this step
+              is not applicable if you don't use SOGo groupware.</li>
 <li>2015-01-13: [All backends] Fixed: Incorrect path of command 'sogo-tool` on OpenBSD.</li>
-<li>2015-01-12: [SQL backends] Fixed: Not apply service restriction in Dovecot SQL query file while acting as SASL server.</li>
+<li>2015-01-12: [SQL backends] Fixed: Not apply service restriction in Dovecot
+              SQL query file while acting as SASL server.</li>
 </ul>
 <h2 id="general-all-backends-should-apply-these-steps">General (All backends should apply these steps)</h2>
+<h3 id="optional-fixed-return-receipt-response-rejected-by-iredapd-plugin-reject_null_sender">[OPTIONAL] Fixed: return receipt response rejected by iRedAPD plugin <code>reject_null_sender</code></h3>
+<p>Note: this is applicable if you want to keep iRedAPD plugin <code>reject_null_sender</code>
+but still able to send return receipt with Roundcube webmail.</p>
+<p>According to RFC2298, return receipt envelope sender address must be empty. If
+you have iRedAPD plugin <code>reject_null_sender</code> enabled, it will reject return
+receipt response. To particularly solve this issue, you can set below setting
+in Roundcube config file <code>config/config.inc.php</code>:</p>
+<ul>
+<li>on RHEL/CentOS/OpenBSD, it's <code>/var/www/roundcubemail/config/config.inc.php</code>.</li>
+<li>on Debian/Ubuntu, it's <code>/usr/share/apache2/roundcubemail/config/config.inc.php</code>.</li>
+<li>on FreeBSD, it's <code>/usr/local/www/roundcube/config/config.inc.php</code>.</li>
+</ul>
+<pre><code>$config['mdn_use_from'] = true;
+</code></pre>
+
+<p>Note: if other mail client applications don't set smtp authentication user as
+envelope sender of return receipt, same issue will occurs. You must disable 
+iRedAPD plugin <code>reject_null_sender</code> in <code>/opt/iredapd/settings.py</code> to make all
+mail clients work.</p>
+<p>iRedAPD plugin <code>reject_null_sender</code> rejects message submitted by sasl
+authenticated user but with null sender in <code>From:</code> header (<code>from=&lt;&gt;</code> in Postfix
+log). If your user's password was cracked by spammer, spammer can use this
+account to bypass smtp authentication, but with a null sender in <code>From:</code>
+header, throttling won't be triggered.</p>
 <h3 id="fixed-incorrect-path-of-command-sogo-tool-on-openbsd">Fixed: Incorrect path of command <code>sogo-tool</code> on OpenBSD</h3>
 <p>Note: this step is applicable to only OpenBSD.</p>
 <p>Please check user <code>_sogo</code>'s cron job, make sure path to <code>sogo-tool</code> command is
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 5d546713..e103cde2 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
@@ -8,12 +8,45 @@ WARNING: This is still a working in progress draft document, do __NOT__ apply it
 
 ## ChangeLog
 
-* 2015-02-02: [All backends] Fixed: Not backup SOGo database. Note: this step is not applicable if you don't use SOGo groupware.
+* 2015-02-04: [All backends] [__OPTIONAL__] Fixed: return receipt response rejected
+              by iRedAPD plugin `reject_null_sender`.
+* 2015-02-02: [All backends] Fixed: Not backup SOGo database. Note: this step
+              is not applicable if you don't use SOGo groupware.
 * 2015-01-13: [All backends] Fixed: Incorrect path of command 'sogo-tool` on OpenBSD.
-* 2015-01-12: [SQL backends] Fixed: Not apply service restriction in Dovecot SQL query file while acting as SASL server.
+* 2015-01-12: [SQL backends] Fixed: Not apply service restriction in Dovecot
+              SQL query file while acting as SASL server.
 
 ## General (All backends should apply these steps)
 
+### [OPTIONAL] Fixed: return receipt response rejected by iRedAPD plugin `reject_null_sender`
+
+Note: this is applicable if you want to keep iRedAPD plugin `reject_null_sender`
+but still able to send return receipt with Roundcube webmail.
+
+According to RFC2298, return receipt envelope sender address must be empty. If
+you have iRedAPD plugin `reject_null_sender` enabled, it will reject return
+receipt response. To particularly solve this issue, you can set below setting
+in Roundcube config file `config/config.inc.php`:
+
+* on RHEL/CentOS/OpenBSD, it's `/var/www/roundcubemail/config/config.inc.php`.
+* on Debian/Ubuntu, it's `/usr/share/apache2/roundcubemail/config/config.inc.php`.
+* on FreeBSD, it's `/usr/local/www/roundcube/config/config.inc.php`.
+
+```
+$config['mdn_use_from'] = true;
+```
+
+Note: if other mail client applications don't set smtp authentication user as
+envelope sender of return receipt, same issue will occurs. You must disable
+iRedAPD plugin `reject_null_sender` in `/opt/iredapd/settings.py` to make all
+mail clients work.
+
+iRedAPD plugin `reject_null_sender` rejects message submitted by sasl
+authenticated user but with null sender in `From:` header (`from=<>` in Postfix
+log). If your user's password was cracked by spammer, spammer can use this
+account to bypass smtp authentication, but with a null sender in `From:`
+header, throttling won't be triggered.
+
 ### Fixed: Incorrect path of command `sogo-tool` on OpenBSD
 
 Note: this step is applicable to only OpenBSD.