One of the goals of Exchange 2010 mailbox resiliency is to minimize data loss. In Exchange 2010 SP1 we added continuous replication block mode to help further reduce data loss when a failover occurs. However, on a very busy mailbox database with a high log generation rate, there is a greater chance for data loss if replication to the passive database copies cannot keep up with log generation.
Read more at source: http://blogs.technet.com/b/exchange/archive/2011/05/06/exchange-2010-mailbox-moves-and-mailbox-resiliency.aspx
Cheers,
Anderson Patricio
http://blogs.msexchange.org/patricio
Twitter: @apatricio