&gt; Probably the biggest thing is that it translates a traditional document-based data structure (mailboxes via IMAP) into a stateless, streaming transactional data structure.<p>So why can&#x27;t this be done on the client side? Are you telling me that to avoid storing more data on the clients, they should give up their privacy to whoever operates the server? Not like Outlook or Thunderbird take a substantial amount of time to do most operations. Is this a good tradeoff for them?<p>I understand, yes, technically adept users can run the server, but there&#x27;s still many who won&#x27;t, who will use your server or someone else&#x27;s server and be required to make this tradeoff.<p>Of the products you mention - yes, yours is the only one I&#x27;d consider and for this very reason, I can run the middle server myself. I just have to think about those people who won&#x27;t do it though.