Exchange Server 2016, RTM as of October 2015, is still very much freshly baked having just come out of the oven from Redmond. Two recent projects that I've worked on have required me to consider deploying it as the “Hybrid” server (not an actual role- I'll get to that later) for integration and coexistence with Office 365 Exchange Online.
With anything new there is a learning curve as to how the new product now works (not that dissimilar from previous versions of Exchange Server) and what will work with the existing environment to not compromise service.
There is an unwritten assumption that is made in our hybrid guidance that you have already properly deployed and completed the coexistence process with the current versions of Exchange in your on-premises environment. - The Exchange Team
Back when Exchange Server Hybrid was first introduced in Exchange Server 2010, there is a special Office 365 Hybrid key that is required to license the server. This is the first confusing part of the Exchange Server Hybrid world. This special key does not mean that you need any additional licenses. It's simply a special and FREE key that the Office 365 team provide to allow for hybrid connectivity to Office 365 / Exchange Online.
The second confusing part about Exchange Server Hybrid Edition is that in Exchange 2010 there was a EMC checkbox to enable “Exchange Server Hybrid” on that server. What this simply did, and has always done on all version of Exchange Server “Hybrid” (2010, 2013 and now 2016), is run a wizard (the Hybrid Configuration Wizard or HCW) which ask users for CAS and Mailbox servers to run some magic background config wizardry on. The reason that it asks for the CAS is so that the receive connectors on these servers can be configured. Then the reason it asks for the Mailbox is to ensure that we properly configure the send connectors. Selecting those servers is not selecting your “Hybrid” servers as such, rather, it is just for mail flow control as well as management tools and integration with Office 365 / Exchange Online.
Therefore the Exchange Hybrid server is nothing more than a standard Exchange server with some select roles required and installed (usually HUB+CAS multi-role deployment in Exchange 2010, CAS+MBX multi-role deployment in Exchange 2013 and MBX in Exchange 2016). There isn't any real or specialised version of Exchange Server Hybrid at all!
Different consultants or professionals look at this in several ways and usually come up with answers like “it depends” or its on a “per circumstance basis”. When I get asked this question I look at one of two scenarios and every other decision is based on this question: will my organisation keep Exchange Server Hybrid in-place on-premises for long term, either for feature requirements or to be the last Exchange Server required for cloud resource management?
So, what does that mean? Exchange Server Hybrid is really necessary for coexistence between on-premises and Exchange Online, smooth transition and migration to Exchange Online from on-premises, as well as managing Exchange Online resources from on-premises while AADConnect is enabled. Apart from that, Exchange Hybrid can be de-commissioned and Exchange Server itself in a very small footprint can be left on-premises in the scenario where AADConnect will be in-place.
Going back to the question, if the organisation I was helping transition to Exchange Online had a requirement that Office 365 Exchange Online resources be managed from on-premises, and there were requirements for things like secure mail or even to keep a handful of mailboxes on-premises, then YES it's important to upgrade Exchange Server Hybrid to Exchange Server 2016.
The migration is not simply to have the latest and greatest as that's the best version. As the snowball effect starts, it begins with Microsoft Support. Microsoft currently have an official N -1 support structure for Exchange Server versions. Snowball that further and that includes direct compatibility with Exchange Server 2016. Exchange Server 2016 will only integrate with Exchange Server 2013 (the hybrid configuration wizard allows for N -2 as well as Exchange Server 2016 itself, supporting Exchange Server 2013 and 2010).
For a long term investment into Exchange Online, so far means, we'll need to maintain a reasonably up to date version of Exchange on-premises (N -1) to keep up with the evergreen Exchange Online. As I mentioned earlier, this could be left as the Hybrid server or simply a small footprint of standard Exchange servers.
Lets flip the coin and look at the other scenarios. Exchange Server Hybrid has been implemented to facilitate a smooth transition to Exchange Online. On-premises administrators love Exchange Online and have no issues with using a web console and remote PowerShell to conduct all administrative tasks. There is also no requirement for AADConnect moving forward.
In this scenario, whatever version of Exchange Server Hybrid is in-place, that is perfectly fine to stay there until transition is completed to Exchange Online*. Notice the * ? There is a small catch in that the only other consideration on this point is that of timing. It is all well and good to say that an organisation does not intend to keep Exchange Server Hybrid in-place more than it's necessary, it's another thing putting that in-practice.
From my experiences in transitioning organisations from SMB to large enterprise to Office 365, the timing is key. A two year transition process for large enterprise (yes I've been involved in lengthy transitions like that) can cause problems with evergreen Office 365 where Office 365 upgraded to a backend based on Exchange Server 2013 over the previous Exchange Server 2010 based backend. Timing shouldn't be that much of a burden, but, it can come into play for those longer running larger projects.
I thought I'd take a quick side step before wrapping up for now and rapid fire some other considerations that I've come across that may well be helpful in deciding to upgrade to Exchange Server 2016 Hybrid:
Every client and environment is different and every case should be considered. Breaking down a large problem into smaller and easier chunks through is key to a positive decision. I like to do this process and have come up with my simple and single question to kick start the process on to or not to upgrade Exchange Server Hybrid.
In the coming weeks I will be putting together a series of blog posts on Exchange Server 2016. The latest and greatest in enterprise messaging platforms has grown up allot and is even easier to transition to than ever before.