title: Excel remove data from 1 file to another
body: <p>I use acymailing to send newsletter.</p>
  
  <p>I have my excel workbook "A" which contains all my users (20 000 people).
  I import in Acymailing and sent a newsletter to 6500 people.
  I get the list of these 6500 people in excel workbook. Let's call it "B".</p>
  
  <p>Now I want to get the list of all the people who didn't receive my newsletter:
  A - B, so at the end I should get 20 000 - 6500 = 13 500 people.</p>
  
  <p>How can I do that?</p>
  
  <p>Someone can help me please?</p>