Nearly two years ago, I presented a session called High Availability Design Considerations at TechEd North America 2010. During my session I described the changes we were making around how MAPI connectivity should occur after mailbox moves and cross-site database *over events. Unfortunately, after I gave that presentation we cut the feature due to the complexity of the changes we needed to introduce and the lack of time for testing prior to Service Pack 1’s release. And though I tried hard to get the work prioritized, we were unable to ship these changes in Service Pack 2, either.
Read more at source: http://blogs.technet.com/b/exchange/archive/2012/05/30/rpc-client-access-cross-site-connectivity-changes.aspx