We have just made a fresh install of a Zimbra multi-server environment in our test lab consisting of three servers. All servers are running:
OS = Ubuntu Server Edition 8.04.2 64-bit
ZCS = Network Edition v5.0.13

All setting are at their default.

Our problem is with Outlook and Outlook Express. They keep giving us the following errors when trying to access the mailbox:

This is the error message we're getting:
An unknown error has occured.

   Account: (server's ip address)
   Server: (server's ip address)
   User name: (username)
   Protocol: IMAP
   Port: 143
   Secure(SSL): 0
   Code: 800ccc03
Apple Mail works without a hitch.

What are we doing wrong?

Thanks in advance.