To stop the ‘empty subject line’ complaint: 

• Right click on the Avast icon in the System Tray at the bottom right of your screen. 
  (If it’s hidden, you may have to click on the button to see it.)
   
• Select On-access Protection Control and click Details>>
  (if you only see << Less Details, you’re already on the right screen)
  
   
• Scroll down and click on the Outlook/Exchange icon in the column on the left.   
  
   
• Press Customise and then click the > arrow at the top right of the tabs until you can see the Heuristics tab, and then click on that. 
  
  
   
• Slide the arrow beyond High to Custom and then click the Customise button. 
  
   
• Untick the Subject structure check option
  (if you also get annoyed by messages about “suspicious whitespace” or “iframes” in your emails, you can disable those here too)
  
   
• Click OK three times to close.  

No other protection is disabled, but blank subjects (and whitespace/iframes if you deselected them) will be ignored from now.