OpenMsgStore returns MAPI_E_FAILONEPROVIDER with Exchange 2013 proxy

Hello

I am working on a new version of some software that uses CreateStoreEntryID t
generate an EntryId for a mailbox and OpenMsgStore to connect to the mailbox.
have recently added AutoDiscover functionality to the software to enabl
connecting to Exchange 2013 mail accounts. When I am connecting to mailboxes o
an Exchange 2010 or 2007 server that gets proxied through an exchange 201
server in the domain I get a MAPI_E_FAILONEPROVIDER back when OpenMsgStore i
called

Any ideas on how I can...

OpenMsgStore returns MAPI_E_FAILONEPROVIDER with Exchange 2013 proxy