It has always been the case that the only supported patch level for Exchange on-premises – whether Exchange 2010, 2013, 2016 or soon 2019 – is n-1. Meaning that although your Exchange organisation doesn’t need to be bang up-to-date to be supported in a hybrid relationship with Exchange Online, you have to remain effectively within 6 months of Microsoft’s quarterly patching schedule to remain in support. Although this doesn’t sound like too much of a challenging task, over the years I have come across organisations time and time again, large and small, who find staying within – or even getting to – this state a challenge. With some saying, “Ah well, we’ll leave it as it is and just patch if we need to raise a call with Microsoft, because that’s almost completely unlikely to happen, and we’re going to be moving all our mailboxes online in any case.”
… Moving on.
Anyway, until recently that was an option for the daring Exchange administrator, who could choose to take the risk, and run the Hybrid Wizard anyway, and continue leaving his Exchange Server un-patched, and risk features breaking or potential security vulnerabilities.
But watch out all you adrenaline junkies out there! The Office 365 Hybrid Configuration Wizard now checks the patch level of your Exchange Server, whether 2010, 2013, 2016 or 2019 and will not proceed if you are running a build older than n-1 (handily informing you which update releases are currently supported).
Although this may be a pain or even a potential blocker to some organisations, I think in the long run this will put those moving to Exchange Online in a much better position to make their migrations and the coexistence period a success, and worth being aware of if you were planning on leaving those Exchange 2010 servers at RTM!