title: Is it possible to attack a user password with known salt
body: <p>I've been told that email is a bad salt, because it's not unique and connected to the user.<br>
  And if a user uses the same password on 2 sites, there will be equal hash. </p>
  
  <p>So, what's wrong with it? what is attack scenario?<br>
  Suppose we have both hash and salt. So, other site has the same hash in their database. 
  How can we do any harm to this user on the other site? Can we at all?</p>
  
  <p>I don't see any possibility, but I am not an expert in security, so, I'd like to hear from ones who are, <em>with practical and concrete answers, of course.</em></p>
  
  <p>I am not going to break anything. I am asking this question in the context of this one: is email or (registration timestamp) a good salt?</p>
  
  <p>Certain and practical answers, please. </p>