On March 17th, Microsoft released Cumulative Update 8 for Exchange Server 2013. By now, we're all used to the idea that cumulative updates and not only Service Packs have also become a vehicle to introduce new features into Exchange. Hence, it is no surprise that CU8 comes with a bunch of new features and improvements alongside a myriad of bug fixes.
It has been since Cumulative Update 5 that Microsoft introduced new "hybrid" features. So you can imagine how pleased I was to learn that CU8 contained a rather important improvement with regards to hybrid deployments.
Before we dive into the feature itself, let me give some background information on the problem the feature will help to solve. A hybrid deployment is often deployed to allow the so-called "hybrid mailbox moves", sometimes you'll also see them referenced as "MRS moves" or "remote mailbox moves". Regardless of what name you use, in my opinion these mailbox moves offer significant value over other migration methods. The simple reason being that hybrid mailbox moves are more resilient, more flexible and almost transparent to the end user. In a staged- or cutover migration, once a mailbox is moved, Outlook's offline cache (.OST file) has to be recreated. While you might think this is not really a problem, try and imagine how that would feel like for an organization that has limited bandwidth but has several hundred gigabyte worth of mailbox data. In such scenario, if you can avoid having to download the data which you just have 'uploaded' to Office 365, then that is something you would want to look into.