The Exchange team just announced the availability of Update Rollup 4 for Exchange 2010. We got a number of MAPI fixes in there:

979810 You cannot connect an Exchange Server 2010 mailbox by using a MAPI client

980353 A MAPI application that is used to access Exchange Server 2010 mailboxes crashes when the application accesses an address book

980354 "MAPI_E_INVALID_PARAMETER" error message when you copy email messages from an Exchange Server 2010 mailbox

980852 The RpcClientAccess process on an Exchange Server 2010 server crashes when you access a mailbox by using a MAPI application

The last one is particularly interesting because this crash was stopping MAPI applications from accessing recovery mailboxes.