Sounds like it's telling you that IMAP cannot connect.
Do you have the IMAP service running on the exchange server? If so, can you successfully open a test connection to it from your workstation?
c:\> telnet [server name] 143
You should get a result something like this:
* OK Microsoft Exchange 2000 IMAP4rev1 server version 6.0.6619.12 (Mail.customer.net) ready.
If the service is up, you might also check for ACLs/firewall rules that might be blocking the server from connecting on port 143
Cliff