Seeking advise for Migration On-prem to Office 365

Copper Contributor



Our company wants to migrate mailboxes on our on-prem (this mailboxes are used by the application) total of 50 mailboxes. Hope you can guide what migration is the best and safe way to use? In my mind is staged as they would like to do testing first on one mailbox to test if it is working fine with the application using the mailbox is it possible ? 


Best Regards,

Mark Diaz

7 Replies


i hope the following link will help you solving your issue!

R, Sameer


Hi Sameer,


May I know what will happen to emails that are being sent to the mailbox during migration?

Do we have to update directly the mx record? will the emails will be forwarded?


Things to consider

Setting up an email cutover migration to Office 365 requires careful planning. Before you begin, here are a few things to consider:

  • You can move your entire email organization to Office 365 over a few days and manage user accounts in Office 365.

  • A maximum of 2,000 mailboxes can be migrated to Office 365 by using a cutover Exchange migration. However, it is recommended that you only migrate 150 mailboxes.

  • The primary domain name used for your on-premises Exchange organization must be an accepted as a domain owned by you in your Office 365 organization.

  • After the migration is complete, each user who has an on-premises Exchange mailbox also will be a new user in Office 365. But you'll still have to assign licenses to users whose mailboxes are migrated.


Impact to users

After your on-premises and Office 365 organizations are set up for a cutover migration, post-setup tasks could impact your users.

  • Administrators or users must configure desktop computers         Make sure that desktop computers are updated and set up for use with Office 365. These actions allow users to use local user credentials to sign in to Office 365 from desktop applications. Users with permission to install applications can update and set up their own desktops. Or updates can be installed for them. After updates are made, users can send email from Outlook 2013, Outlook 2010, or Outlook 2007.

  • Potential delay in email routing        Email sent to on-premises users whose mailboxes were migrated to Office 365 are routed to their on-premises Exchange mailboxes until the MX record is changed.


How does cutover migration work?

The main steps you perform for a cutover migration are shown in the following illustration.

Process for performing a cutover email migration to Office 365

  1. The administrator communicates upcoming changes to users and verifies domain ownership with the domain registrar.

    See how-to steps in Verify you own the domain.

  2. The administrator prepares the servers for a cutover migration and creates empty mail-enabled security groups in Office 365.

    See how-to steps in Prepare for a cutover migration.

  3. The administrator connects Office 365 to the on-premises email system (this is called creating a migration endpoint).

    See how-to steps in Connect Office 365 to your email system.

  4. The administrator migrates the mailboxes and then verifies the migration.

    See how-to steps in Migrate your mailboxes.

  5. Grant Office 365 licences to your users.

  6. The administrator configures the domain to begin routing email directly to Office 365.

    See how-to steps in Route your email directly to Office 365.

  7. The administrator verifies that routing has changed, and then deletes the cutover migration batch.

    See how-to steps in Delete the cutover migration batch.

  8. The administrator completes post-migration tasks in Office 365 (assigns licenses to users and creates an Autodiscover Domain Name System (DNS) record), and optionally decommissions the on-premises Exchange servers.

    See how-to steps in Complete post migration tasks.

  9. The administrator sends a welcome letter to users to tell them about Office 365 and to describe how to sign in to their new mailboxes.

Hi @Mark Louie Diaz,


What is your Exchange version ? The best way to migrate is using Hybrid and secure, you can always rollback.

@Mark Louie Diaz If you are running On-premise exchange 2010 and above then you are going to be blessed with Hybrid configuration mode. 


You can run Deployment assistant tool to validate the scenarios for Hybrid configuration 

Thanks for the reply Nuno,


If we choose Hybrid and after migration can we decommission the on-prem? We have 2010 Exchange btw.

Hi @Mark Louie Diaz you can continue to use Hybrid and migrate your Exchange to 2016 you can request a key here and you will need to continue to have hybrid implemented to be full supported if you have directory synchronization.