Yes, there were major changes between the way IMAP worked in Outlook 2003 & Outlook 2007 too, I think they re-wrote their IMAP code as it behaved completely differently. Admittedly it was much better, but it broke our custom IMAP server which I ended up fixing. I can't remember exactly why now, but Outlook 2003's implementation was bizarre, as if they'd not read the RFC.

That reminds me of the bizarre big because they used a short uint to store the message UID. Maybe it wasn't a short but it definitely wasn't 32-bit as per the spec, there was some magic number that if you went over 'boom'. As an end user it appeared that some messages just disappeared.