title: How do I fix "501 Syntactically invalid HELO argument(s)"?
body: <p>I'm using exim on both the sending and relay hosts, the sending host seems to offer:</p>
  
  <pre><code>HELO foo_bar.example.com
  </code></pre>
  
  <p>Response: </p>
  
  <pre><code>501 Syntactically invalid HELO argument(s)
  </code></pre>