Some rules in Exchange can be set to allow email from either authenticated users or only for an account that exists in the Exchange database...
Make sure you also have a transport rule that would accept a connection from the IP address of your AC server...
You can also use this article to troubleshoot: How to Use Telnet to Verify Basic IMAP Connectivity in Exchange 2007: Exchange 2007 Help
(should be pretty much the same commands no matter which version of Exchange you have).