I don’t use Outlook Express, but some of my customers do. In my experience it has displayed unpredictable IMAP support.
I just read this in the Dovecot (an awesome IMAP server) configuration file about some workarounds:
# delay-newmail: # Send EXISTS/RECENT new mail notifications only when replying to NOOP # and CHECK commands. Some clients ignore them otherwise, for example OSX # Mail (<v2.1). Outlook Express breaks more badly though, without this it # may show user "Message no longer in server" errors. Note that OE6 still # breaks even with this workaround if synchronization is set to # "Headers Only".