title: Exim - A lot of frozen messages without sender
body: <p>I'm new to Exim and I'm using it as a smarthost (to recive emails from users and send it to my ISP).
  <img src="https://i.stack.imgur.com/7rSwP.png" alt="enter image description here"></p>
  
  <p>The system is working ok, but I have some problems with frozen messages. I watch the queue sometimes and there are some frozen messages without sender, so Exim cannot send them, because if there isn't sender it can't authenticate with the ISP.</p>
  
  <p>Where can I start to debug this?</p>
  
  <p>Thanks a millon.</p>