title: How reliable are IMAP UIDs?
body: <p>I'm writing an IMAP crawler, which is part of an Open Source app, and for incremental crawling I'd like to use the message UIDs to find out whether I've already seen a particular message or not.</p>
  
  <p>Now I found <a href="http://mailman2.u.washington.edu/pipermail/imap-protocol/2007-March/000436.html" rel="nofollow">a post</a> from 2007 that says IMAP UIDs aren't always reliable. More specifically, if the flag UIDNOTSTICKY is set, you can't assume the UIDs are always the same.
  So my question is, do I have to take the case UIDNOTSTICKY=true seriously, or are IMAP servers with non-sticky UIDs legacy stuff and not in (widespread) use anymore?</p>