title: What causes this very unusual SMTP error?
body: <p>Sending SMTP mail via an Office 365 mail server, using .NET <code>System.Net.Mail.SmtpClient</code> I'm getting the following error:</p>
  <pre><code>The server response was: 4.3.21 Mailbox server busy; mail intentionally delayed
  </code></pre>
  <p>I can't find any reference to this specific code anywhere - the closest thing I can find is <a href="https://docs.microsoft.com/en-us/exchange/troubleshoot/email-delivery/remote-server-returned-mailbox-busy" rel="nofollow noreferrer">4.3.2 mailbox busy</a></p>
  <p>It seems to be a transient error, but I'm getting a lot of them.</p>
  <p>Any idea what the code means?</p>