title: Message ID in a POP3 protocol
body: <p>my task is to retrieve a set of messages from GMail via POP3 (no IMAP).
  I can do RETR MSG #, and it is prohibited to delete.</p>
  
  <p>Fetchmail and procmail are constantly trying to download a same set of new, unread messages (this part goes to ServerFault). Is there a header specially designed to distinguish previously read messages? Or I should do checksum of message body/subject/date?</p>