IMAP4

I have a vendor who wants to connect to our Exchange 2013 setup using IMAP4. I have set the IMAP services on the CAS and mailbox servers to automatic and started them. However, if I test IMAP connection it fails telling me "Log onto incoming mail server (IMAP): General authentication failed. None of the authentication methods supported by your IMAP server (if any) are supported on this computer."

I am testing this from a Windows 7 machine using Outlook 2010, the test account has IMAP connection enabled. What have I missed?