title: SMTP Error 503 - All RCPT commands were rejected
body: <p>I'm trying to host multiple Laravel applications on cPanel and I have been facing this issue for the past 3 days:</p>
  <pre><code>503-Please turn on SMTP Authentication in your mail client.  (domain.com)
  503-[XXX.XXX.XXX.XXX]:XXXXX is not permitted to relay through this server
  503-without authentication.
  503 Valid RCPT command must precede DATA
  </code></pre>
  <p>The websites are currently hosted on a server that runs CentOS 7 with WHM cPanel. The mail are sent via SMTP Relay using Microsoft Exchange Server.</p>
  <p>This is the current email configuration that I'm using in Laravel:</p>
  <pre><code>MAIL_DRIVER=smtp
  MAIL_HOST=host.domain.com
  MAIL_PORT=25
  MAIL_USERNAME=null
  MAIL_PASSWORD=null
  MAIL_ENCRYPTION=null
  MAIL_FROM_ADDRESS=&quot;info@domain.com&quot;
  MAIL_FROM_NAME=&quot;Example&quot;
  </code></pre>
  <p>The steps that I have tried:</p>
  <ul>
  <li>Tried sending mail via Laravel's Tinker using <code>Mail::raw()</code> and it works. Weird but still don't understand how does that work.</li>
  <li>Able to send emails via <code>telnet</code></li>
  </ul>
  <p>I'm pretty sure that I'm missing some steps here and I'd be glad if anyone can point it out for me.</p>