From my understanding, Exchange Server 2010 was released on the 9th of November 2009 - and Exchange Online during the middle of 2011 - just under 2 years since the original RTM of Exchange 2010 was made available. From the above, I've come to the conflusion that Remote Mailboxes must have been added to Exchange Server 2010 as part of one of its updates - but I haven't been able to find any documentation online that gives any exact dates as to when this functionality was introduced. Would somebody be able to confirm when this was added?
SnapOverflow
Based on my research, Remote Mailbox is a new feature in Exchange 2010 SP1.
More details about it: Support for Hybrid Deployments with Exchange Online