Recently I had an issue during an EX2003 - EX2010 migration where MAPI appeared not to work but Outlook Anywhere did!!
The symptoms were this:
An outlook profile would refuse to connect first time but upon a restart of Outlook you would get a standard username/password dialog box. Once the credentials were entered Outlook would connect and the user would be happy. A quick look at the connection status showed that Outlook was connected over HTTPS ie. Outlook Anywhere - But this had not been configured for the client so how was this happening?
One look at the Outlook profile showed that Outlook Anywhere was indeed enabled, indeed if you disabled this it would immediately re apply itself next time Outlook was launched.
All the ususal suspects were examined in turn as we thought it must be a GPO or dodgy Outlook .PRF file, the onsite guys even thought it was some new Exchange 2010 security feature that forces all connection to be made by HTTPS!!
The Exchange CAS array was behind a hardware load balancer so this was bypassed but the result was the same. So that ruled out the load balancer...So we thought :-)
Basically I had made a schoolboy error and had neglected to add the RPC Endpoint Mapper (TCP:135) to the Exchange Virtual IP. The result was that Exchange treated all client connections as remote and using a bit of autodiscover wizardry it established a HTTPS connection and modified the Outlook profile to reflect a HTTPS connection.
Closer examination of the load balancer showed the error and once RPC traffic was added to the load balancer front end everything started working properly.
But why did it still fail when I bypassed the loadbalancer? Simple - I had a peaky hosts file directing the CAS array name to the load balancer so changing the CAS array 'A' record had no effect!
Moral of the story is to check the simple things first I guess!
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