title: Mandrill: Queued mail for delivery, but 550 Bad Mailbox
body: <p>I use <strong>Mandrill</strong> to send my transactional emails. Several days ago I started to analyse bounces and found some interesting reports. Some of my hard bounces have  following messages simultaneously:</p>
  
  <ul>
  <li><code>Queued mail for delivery</code></li>
  <li><code>smtp;550 5.1.1 RESOLVER.ADR.RecipNotFound; not found</code></li>
  </ul>
  
  <p>It seems that mailbox is invalid. To prove this, I tried to send an email using terminal:</p>
  
  <pre><code>$ telnet mx.example.com 25
  Trying 100.100.100.100...
  Connected to mx.example.com.
  Escape character is '^]'.
  220 mx.example.com Microsoft ESMTP MAIL Service ready at Wed, 6 May 2015 20:01:56 +0300
  HELO mx.example.com
  250 mx.example.com Hello [200.200.200.200]
  MAIL FROM: &lt;noreply@mysite.com&gt;
  250 2.1.0 Sender OK
  RCPT TO: &lt;user@example.com&gt;
  250 2.1.5 Recipient OK
  DATA
  354 Start mail input; end with &lt;CRLF&gt;.&lt;CRLF&gt;
  from: noreply@mysite.com
  to: user@example.com
  subject: test   
  
  Hi!
  .
  250 2.6.0 &lt;99534bb0-ed72-428b-a487-48aaac32ca11@tmh-srv080.hq.tmh.local&gt; [InternalId=727698] Queued mail for delivery
  QUIT
  221 2.0.0 Service closing transmission channel
  Connection closed by foreign host.
  </code></pre>
  
  <p>So, it seems, <strong>Microsoft ESMTP MAIL Service</strong> does not check recipient, but push my message into the queue. Actually I could not get 550 code from mail server in terminal, but <strong>Mandrill</strong> could.</p>
  
  <p>Any ideas how it is done in <strong>Mandrill</strong>? Is it possible to validate email when server uses <strong>Microsoft ESMTP MAIL Service</strong>?</p>