title: MailKit IMAP fetch only new, not downloaded messages
body: <p>i'm using MailKit to implement an IMAP email client. In the various examples i've seen that the code to fetch message headers is this one:</p>
  
  <pre><code>var messages = client.Inbox.Fetch (0, -1, MessageSummaryItems.Full | MessageSummaryItems.UniqueId).ToList();
  </code></pre>
  
  <p>If i have correctly understood, this fetches always ALL messages.</p>
  
  <p>My idea is to save in a local db messages already fetched, and then, for subsequent fetches, getting only differences.
  Is there a way to accomplish this?
  Thanks</p>