I have a client running Exchange 2010, 3 mail stores and 300 mailboxes.

We are in the process of updating their infrastructure and as part of it are moving them to 365 for their mail.

At the same time, we are moving them away from a data centre hosting to internal hosting and the plan is to not bring the Exchange server across.

Hoping for advice on the best way to handle the migration to 365. I'm in two minds, the cutover method looks the best and neatest but from reading up the wizard limits it to 150 mailboxes, but if using EAC, the limit is 2000. Unfortunately, being Exchange 2010, they do not have EAC. Are they limited to the 150 through the wizard as a hard limit or are we able to run it twice to get all mailboxes?

The second option is hybrid, but as we are not planning on having a local exchange instance anymore this would be less desirable. I understand having a local instance of Exchange will allow for better administration of the 365 side so I'm not completely against running up a 2016 instance in the new environment.

As it stands, I have 365 up and running, Exchange is up to date, I haven't got the ADSync configured as yet (waiting on which method!) and I'm looking for any advice!



  • What else do you have on-premises? If you have sharepoint or other applications that need an Exchange host on-prem, this can force you to hybrid. SMTP relay from applications can be a pain in O365 and has limitations. Hybrid might be able to make this easier on you to keep the applications you have today working. – blaughw Sep 16 '16 at 16:46

I only do hybrid.

Other methods do not even enter the equation. Things can get in the way, you can find issues, the client can even change their mind!

Hybrid allows you to test everything as you go along, introducing things slowly and ensure that the business needs are met. Takes the pressure off!


With 300 users you can really go either way, technically. It really depends on your end goals.

If you want to get rid of Exchange, then you are looking at what is called "cloud-only deployment". I would recommend not configuring Directory Sync in this scenario, it will almost assuredly cause you problems after you remove it later. I would bulk import and create your users and use Cutover migration in this case.

  • It's not that complicated
  • Manage users in the cloud
  • Exchange Server / Sync Server not required


  • You need to recreate or manually copy some things like Distribution groups.
  • Separate Logons between local resources and O365
  • Manually reconfigure desktop and mobile devices

Hybrid migration gives you more flexibility and it works very well. But this does require Directory Sync be configured. If you are running Directory Sync, you also need an Exchange server around for management purposes.


  • Users and Groups are pre-created in cloud
  • Usernames/Passwords are same both locally and in O365
  • You can move mailboxes back from O365
  • Clients that support autodiscovery will pick up changes automatically.


  • You really do need to keep at least one Exchange server around for management purposes
  • Somethings don't work as you would like (Group Management is a big one).
  • You need to Keep Azure AD Connect, and Exchange updated, and the OS they are installed on of course.
  • Mixed sources of authority - Office 365 groups in O365, but security and distribution groups in AD (you manage these differently based on SOA).

Exchange purist will scoff, but I usually consolidate the Azure AD Connect and Exchange server onto a single box. At the end of the migration, there are no mailboxes hosted on Exchange, the purposes are now mostly for management, and in a lot of cases to relay mail from internal services (applications, printers, scanners, etc). You may want to check out this resource here: https://technet.microsoft.com/en-us/exdeploy2013/Checklist (if you have trouble with link, search for "exchange server deployment assistant")

If you go the Hybrid route, AFTER migration consider adding in a 2013 or 2016 Exchange server and decommissioning the 2010 server. The interface is closer to Office 365. Further, it can provide a better experience for users that must manage groups if you create a role and grant them access to do so thru 2013/2016 ECP (You can't manage it in Outlook any longer). The alternatives, are command line, ADUC, or manually moving groups to Office 365. See this support article for more info: https://support.microsoft.com/en-us/kb/2417592 (or search for 'Office365 Hybrid group management')

  • The decision to go with Cloud identity or Synchronized identity vs Federated identity should really be made with input from the business. If you have an older user base, SSO might be a requirement. If you're an army of one at an organization, supporting Federation might not be something you want to tackle. – blaughw Sep 16 '16 at 16:49

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