title: Django smtplib.SMTPRecipientsRefused: Recipient address rejected: User unknown in virtual mailbox table
body: <p>I have a django app that requires users to create an account before they are able to use the service. Upon registration, the user is sent an email with a link to activate their account. Lately however, I have noticed that when someone creates an account with slightly more unusual email addresses than gmail.com, an error is being thrown that the user is 'unknown in the virtual mailbox table (This happens in local, dev and in production). This used to not be the case, but i dont understand what has changed. Full error below (with email address anonymized):</p>
  <pre><code>File &quot;/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/smtplib.py&quot;, line 881, in sendmail
      raise SMTPRecipientsRefused(senderrs)
  smtplib.SMTPRecipientsRefused: {'testing@xxxx.com': (550, b'5.1.1 &lt;testing@xxxx.com&gt;: Recipient address rejected: User unknown in virtual mailbox table')}
  </code></pre>
  <p>Do you guys have an idea?
  Thanks!</p>