As part of an Office 365 deployment, you can migrate the contents of user mailboxes from a source email system to Office 365. When you do this all at one time, it's called a cutover migration.

Reason for cutover migration

Choosing a cutover migration is suggested when:
  • Your current on-premises Exchange organization is Microsoft Exchange Server 2003, Microsoft Exchange Server 2007, Microsoft Exchange Server 2010, or Microsoft Exchange Server 2013. /li>
  • Your on-premises Exchange organization has fewer than 2,000 mailboxes.
  Note
Even though cutover migration supports moving up to 2000 mailboxes, due to a length of time it takes to create and migrate 2,000 users, it is more reasonable to migrate 150 users or less.

How does cutover migration work?

Cutover Migration is one of the types of exchange migration where all on-premise exchange server content is moved in a single batch to cloud O365 environment. It’s one of the good plans for those organization who wants to move entire mail environment to the cloud O365 environment. Cutover migration can migrate to 2,000 mail users from an on-premise environment to Office 365 environment. This migration is suitable for exchange server 2003,2007,2010,2013. Pre-requisites steps for migration:  You can migrate up to 2,000 mailboxes. Outlook anywhere should be enabled on the on-premise Exchange server.
  • Third Party Certificate 
  • Please note the Windows Azure Active directory sync tool should be disabled. Otherwise, the cutover migration tab will be showing as greyed out. 
  • UM (Unified messaging) should be disabled. 

Migration Process:

  1. Firstly you need to create the Migration endpoint by clicking on three dots option and selecting Outlook anywhere option on cloud O365 Exchange portal. 
  2. Then you need to fill the email address for on-premises. 
  3. On-premises admin credentials and admin must have access to the below: 
  • Where you need to give the full access to all the mailboxes to be migrated. 
  • Or receive as access permission on the mailbox database. 
If you still not able to connect you can still do it manually by entering below-mentioned details:
  • FQDN of the mailbox server. 
  • FQDN of the CAS server. 
 
  Note
Incremental Sync always works after 24 hours. New emails or deleted emails will get synchronized in every 24 hours. 

From https://support.office.com/en-us/article/Cutover-migration-to-Office-365-9496e93c-1e59- 41a8-9bb3-6e8df0cd81b4

  1. The administrator communicates upcoming changes to users and verifies domain ownership with the domain registrar.
  2. The administrator prepares the servers for a cutover migration and creates empty mail-enabled security groups in Office 365.
  3. The administrator connects Office 365 to the on-premises email system (this is called creating a migration endpoint).
  4. The administrator migrates the mailboxes and then verifies the migration.
  5. Grant Office 365 licenses to your users. 
  6. The administrator configures the domain to begin routing email directly to Office 365.
  7. The administrator verifies that routing has changed, and then deletes the cutover migration batch.
  8. The administrator completes post-migration tasks in Office 365 (assigns licenses to users and creates an Auto discover Domain Name System (DNS) record), and optionally decommissions the on-premises Exchange servers.
  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.