Discussion – Removal of Last Exchange Server in Hybrid Configuration

Every now and then there would be a discussion between Management and Exchange Engineers that can we remove our last exchange server because all mailboxes have been moved to Exchange Online.

I have personally faced this questions and always heard different views from others. I have my own thoughts about it but will share the extracts from Tech Community Discussion. I will be putting the text as it is from the thread.

https://techcommunity.microsoft.com/t5/Office-365/Remove-On-Premises-exchange-Hybrid-and-go-fully-Online/td-p/143255

Keep Last Exchange Server.

• Microsoft FastTrack came back stating that if we do the O365 with only an AAD Connect in place and no exchange server then it puts us in an Un-Supported platform when you call Microsoft for any help.
• Have exchange installed at-least to make sure that your Schema supports the right attributes and that the exchange server should be used to provision the mail enabled accounts so that the right attributes are synced to the cloud.
• Suggestion is to leave the hybrid ON. It can also be used as an email relay within the organization. You can trim down the hardware and give just the bare necessary requirements in it.
• Here is one assumption that Microsoft goes around updating exchange server versions behind the scenes for all the client tenants. They may introduce new attributes (perhaps?) that only Active Directory may not house. I am talking about msExch attributes which is a big deal.
• it is pretty clear at the moment that maintaining one last Exchange server just for management purposes is the supported way to go when you like to synchronize your active directory users and their attributes to Azure AD.
• Sure many guys are going to say that you can use ADSI, third-party tools or even nothing to manage your Exchange users in Office 365 BUT the question is, is it really bothering you to keep a last virtual machine with 2 CPUs and 4 GB RAM to be in a supported scenario for your business critical application like mailing ? It will be also more comfortable for your exchange administrators or even just system administrators to manage your exchange objects, even those are in the cloud, Office 365, or on-premises like function mailboxes. That on-premises server could also be used as SMTP server for on-premises devices like FAX or printers or even on-premises applications that need an SMTP server to send e-mails, think about your NAS System, your firewall etc.
• If you had Exchange Server in your on-premises AD then your AD objects have exchange attributes. That means that you are going to have conflicts if you don’t follow the best practices regarding a hybrid exchange environment.
• Email policies (automatically assign addresses) is a feature only available in on-prem Exchange. I would like to see this feature also in EOL – at least for pure-cloud environment. Anyways, if you remove on-prem Exchange server, you will loose the email policies feature.
• As said, the caveat is when you use adconnect to sync objects – making it more troublesome to change attributes and settings related to mail, distribution group membership without an on premises exchange server.

Remove Last Exchange Server.

• It is a bit different but in practice that is irrelevant. Basically all you need is proxyAddresses attribute, which is included “normal” AD schema.
• it is not exactly that way. Unsupported is only the scenario where you have Azure AD Connect tool synchronizing your on-premises Active Directory objects to Azure AD and also those objects are mail-enabled objects. If you don’t synchronize your on-premises AD objects, for password sync etc, then you can just remove the last Exchange Server.
• Should I encounter any insurmountable problems, I guess I can always simply add an Exchange server back to our domain. 
• Keeping a resource hog and patch management nightmare like Exchange server around in order to manage my cloud email accounts seems to defeat the entire purpose of moving to the cloud in the first place.
• If on the other hand, you would like to go FULL Cloud there is also an option for “small” companies called Microsoft 365 Business. With that license you can join your devices to Azure AD, your mailboxes are hosted in the cloud, you don’t have to synchronize anything and you can manage your computers and devices through Microsoft Intune. Almost no server at all on-premises, but again, it depends on your environment, the use case and what are you trying to achieve.

At the end you can see there are more points in favour of keeping last exchange server and that is my thought as well but it depends on different business goals and scenarios hence take your call based on above points.

Thank you for reading !