Forum Discussion
What is required to decommission the last on-prem Exchange server?
Follow the official guidance here: https://technet.microsoft.com/en-us/library/dn931280(v=exchg.150).aspx?f=255&MSPPError=-2147217396
Pay special attention to the discussion around DirSync - if you decide to keep DirSync, you have to keep the Exchange box for management purposes if you want to stay in supported configuration.
As to what to do with the remaining mailboxes, it's up to you. Just don't "store" them as PSTs... :)
If everything is well documented (as it should), someone leaving an organization is not a problem. Managing your own on-prem Exchange is a more complex task requiring expertise and time and more vulnerable to employees leaving. We used outsourced on-prem Exchange and it was still challenging with experts managing it (especially with updates breaking it). As our agreement came to end, we would have to purchase (maybe it's free for this), install and maintain our own Exchange just to manage a few things. We also want to move to cloud only eventually. So it wasn't feasible for us. One has to follow procedures not to create duplicates, and even if created, it will produce sync errors and they will give insight that something has to be fixed. Already had an issue with duplicated alias. It was a minute fix. As i've said our requirements are simple. Maybe some tasks do require Exchange (you mentioned archiving) or some orgs already have it installed in their environment and are used to work with it, but it sounds like an overkill solution for me (it also might be some admins don't want to let it go). So, i hope MS will come up with a supported scenario without it.
wroot it's not about what's "possible", but what's "supported". If you know what you are doing, you will be perfectly fine with managing things via ADUC/PowerShell or even the old school command-line tools, but if you run into any issues, Microsoft will simply deny you support. I know a lot of organizations that never had Exchange (moved from Lotus Notes for example), and they often choose to simply extend the AD schema with the Exchange attributes and continue managing the objects using the AD tools. Again, it's perfectly doable, but not a "supported configuration", so you are on your own if something happens.
Btw, recently i had a support ticket because of a mysterious smtp address in our contacts which was tied to some missing mailbox. Support helped with this without i think even asking whether we have on prem Exchange. I don't remember them asking about it at all. Well, maybe our tickets so far didn't require it.
Yes, i know this is not supported, that's why i said that i hope this will change. Because it seems impractical to use Exchange Online but having to keep/install standalone Exchange to do managing.