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Recently, I had to deal with a serious problem: backscatter.
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<h1 class="post-title-main">Blocking Invalid Recipients Before They Reach Your Exchange Server</h1>
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August 1, 2025
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4 min
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<div class="post-content-main">
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<p>Recently, I had to deal with a serious problem: <strong>backscatter</strong>.<br>
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One of our mail gateways ended up listed on the <strong>backscatter.org</strong> blacklist for sending bounce messages to forged senders.</p>
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<p>After checking the logs, I quickly realized that our system wasn’t actually protected against backscatter attacks, so I had to do something about it.</p>
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<h2 id="what-even-is-backscatter">What Even Is Backscatter?</h2>
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<p>Backscatter is unwanted email that your mail server sends <strong>after</strong> receiving a message, usually in the form of a <strong>non-delivery report (NDR)</strong> or <strong>bounce</strong> to a <strong>forged sender address</strong>.</p>
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<p>It happens when:</p>
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<ul>
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<li>A spammer sends email with a <strong>fake “From” address</strong> (often an innocent third party).</li>
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<li>Your mail server <strong>accepts</strong> the message first, but later discovers it’s undeliverable.</li>
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<li>Your server sends a <strong>bounce</strong> to the forged address, hitting an innocent person instead of the spammer.</li>
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</ul>
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<p>This makes your server appear to be sending spam, even though you’re just bouncing bad mail.</p>
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<h2 id="the-problem-in-our-setup">The Problem in Our Setup</h2>
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<p>Our mail gateway mostly relays mail to multiple internal Exchange Server clusters.<br>
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We have a list of valid domains configured, so Postfix will only accept mail for those domains.</p>
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<p><strong>The problem?</strong><br>
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Postfix didn’t know which individual recipients were valid on the downstream Exchange servers.<br>
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That meant it would happily accept messages for <strong>nonexistent users</strong>, only to later bounce them, classic backscatter behavior.</p>
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<h2 id="the-goal">The Goal</h2>
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<p>We needed to reject mail <strong>during the SMTP session</strong>, ideally right after the <code>RCPT TO</code> command, if the recipient didn’t exist on the Exchange servers.</p>
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<p>That way, the sending server would get the rejection immediately, and we would never have to generate a bounce message.</p>
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<h2 id="possible-solutions-i-considered">Possible Solutions I Considered</h2>
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<h3 id="1-relay_"><strong>1. relay_recipient_maps</strong></h3>
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<p>This would require maintaining a <strong>full list of valid recipients</strong> in a Postfix lookup table.<br>
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It works well, but means writing and maintaining a script to <strong>sync the list from Active Directory</strong> on a regular basis.</p>
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<p>For us, this was too much custom scripting, too fragile, and too messy to maintain.</p>
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<h3 id="2-virtual_"><strong>2. virtual_mailbox_maps with LDAP</strong></h3>
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<p>Another approach would be using <strong>LDAP lookups</strong> directly against Active Directory to verify recipients in real-time.</p>
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<p>This can work in some setups, but:</p>
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<ul>
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<li>It adds complexity and dependencies.</li>
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<li>It can introduce security concerns.</li>
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<li>It didn’t fit well with our environment.</li>
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</ul>
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<p>So I ruled it out.</p>
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<h2 id="the-solution-reject_unverified_recipient">The Solution: <code>reject_unverified_recipient</code></h2>
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<p>While reading through the <strong>Postfix Address Verification Howto</strong>, I came across the <code>reject_unverified_recipient</code> option - bling! - exactly what I needed.</p>
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<h3 id="how-it-works"><strong>How It Works</strong></h3>
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<p>When an incoming SMTP session reaches the <code>RCPT TO</code> stage:</p>
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<ol>
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<li>Postfix checks if <code>reject_unverified_recipient</code> is enabled for the recipient domain.</li>
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<li>If yes, it temporarily <strong>probes the downstream mail system</strong> to see if the recipient address exists.</li>
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<li>If the downstream system says:
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<ul>
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<li><strong>User exists</strong> → Postfix continues processing.</li>
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<li><strong>User does not exist</strong> → Postfix <strong>rejects immediately</strong> with:
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<div class="highlight"><pre tabindex="0" class="chroma"><code class="language-fallback" data-lang="fallback"><span class="line"><span class="cl">550 5.1.1 <user@example.de>: Recipient address rejected: User unknown
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</span></span></code></pre></div></li>
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</ul>
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</li>
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</ol>
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<p>Because the rejection happens <strong>during SMTP</strong>, no bounce is generated, and backscatter is avoided entirely.</p>
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<h2 id="implementation-in-ispconfig">Implementation in ISPConfig</h2>
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<p>In our case, the mail gateways run <strong>Postfix</strong> with <strong>ISPConfig</strong> as the management interface.<br>
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I implemented a new configuration option in ISPConfig for <strong>per-domain control</strong> of <code>reject_unverified_recipient</code>, along with a <strong>validation server</strong> to specify for the downstream validation server.</p>
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<h3 id="specifying-a-validation-server-for-recipient-verification">Specifying a Validation Server for Recipient Verification</h3>
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<p>One important detail is that Exchange servers cannot validate recipients over the default SMTP transport on port <code>25</code>. To enable recipient validation, you need to activate it on the Exchange server and use the Hub Transport service, which by default runs on port <code>2525</code>.</p>
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<p>Make sure to restrict access to this port, as it requires anonymous login specifically for recipient validation, you don’t want that exposed broadly.</p>
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<blockquote class="blockquote-regular">
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<p><strong>Note:</strong> This setup does not affect your regular mail flow. Only the SMTP probes used for verifying recipients are sent to this validation server.<br>
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Postfix enables this behavior with the <code>address_verify_transport_maps</code> option.</p>
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</blockquote>
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<p>To optimize performance and reduce unnecessary verification probes for addresses that have already been checked, I configured a local cache on the mail gateway using the <code>address_verify_map</code> option. This way, repeated probes for the same recipient are avoided.</p>
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<p>Now, for domains that need recipient verification, we can enable it in the panel and point Postfix to the appropriate <strong>validation transport</strong>.</p>
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<h2 id="the-result">The Result</h2>
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<p>After enabling <code>reject_unverified_recipient</code> and pointing it at the Exchange clusters, the backscatter stopped completely.<br>
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We were no longer accepting messages for invalid recipients.</p>
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<h2 id="conclusion">Conclusion</h2>
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<p>If your Postfix server is acting as a relay for Exchange (or any downstream mail system) and you’re struggling with <strong>backscatter spam</strong>, enabling <code>reject_unverified_recipient</code> can be a clean and effective fix.</p>
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<p>It avoids maintaining large static recipient maps, works dynamically, and ensures that invalid mail is rejected <strong>before</strong> it ever gets into your system.</p>
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