<i>The UIDL extension adds a &quot;UID&quot; but it&#x27;s just an MD5 hash of the contents -- meaning that multiple copies of a message appear to be the same message.</i><p>That sounds like a server problem; to quote the RFC,<p><i>The server should never reuse an unique-id in a given maildrop, for as long as the entity using the unique-id exists.</i><p>Why would you hash the contents? The arrival time should be unique, assuming no two messages could arrive at exactly the same time. That doesn&#x27;t require <i>any</i> hashing.<p>I think POP could&#x27;ve been far better designed, without growing into the complexity of IMAP, with just a few little changes like this.