Types of Exchange Online Migration
1. Staged Migration
For legacy Exchange server versions (2003-2007), the only Microsoft-supported migration method to O365 is Staged Migration. With this Office 365 migration type, you can move your entire email infrastructure in batches. This method is beneficial for legacy Exchange servers if you have more than 2000 seats; however, for a successful migration, some critical factors need to be taken into consideration:
You must synchronize accounts between your on-premises Active Directory domain and Microsoft 365 or Office 365 by using Azure Active Directory sync for a staged migration to work. The primary domain name used for your on-premises Exchange organization must be a domain verified to your Microsoft 365 or Office 365 organization. You can migrate only user mailboxes and resource mailboxes. Other recipient types, such as distribution groups, contacts, and mail-enabled users, are migrated to Office 365 or Microsoft 365 through the process of directory synchronization.
Office messages aren't migrated with user mailboxes. If a user turns on the Out of Office feature before the migration to Office 365, the feature will remain enabled on the migrated mailbox, but the Out of Office message is blank. As a result, people who send messages to the mailbox won't receive an Out of Office notification. To allow Out of Office notifications to be sent, the user needs to recreate the Out of Office message after the mailbox is migrated. If you have previously limited the connections to your source email system, it's a good idea to increase them to improve migration performance. Typical connection limits include client/server total connections, per-user connections, and IP address connections on either the server or the firewall. More details about staged migrations are available here.
2. Cutover / Express Migration
For organizations with less than 2000 users, the easiest, most convenient, and probably the most cost-effective method of migration to Office 365 is called Cutover Migration.
As the name implies, cutover means you can cut overall your on-premise email infrastructure and user accounts in one go. This sort of Office 365 migration process can be used if the email infrastructure runs on Exchange versions from 2003 to 2013 (Note: It is not supported for recent versions).
As you can see from the number of supported users, this would be an ideal solution for small/medium scale organizations. Although you can cut overall your users at once for a successful and less trouble-prone migration experience, Microsoft advised migrating less than 150 user accounts at a time. So, for a user base of 2000, you will need to create 15-20 batches. For successful migration to Office 365, you should keep the following in mind whilst planning:
You can move your entire email organization to Microsoft 365 or Office 365 over a few days and manage user accounts in Microsoft 365 or Office 365. A maximum of 2,000 mailboxes can be migrated to Microsoft 365 or Office 365 using a cutover Exchange migration. However, it is recommended that you only migrate 150 mailboxes at a time. The primary domain name used for your on-premises Exchange organization must be an accepted domain owned by you in your Microsoft 365 or Office 365 organization. After the migration is complete, each user who has an on-premises Exchange mailbox also will be a new user in Microsoft 365 or Office 365. However, you must still assign licenses to users whose mailboxes are migrated. More details about this sort of migration can be found here
3. Hybrid Deployment
The third method of migrating on-premises email infrastructure into Office 365 is via the Hybrid deployment method. This is the most complex and probably the most expensive type of Office 365 migration; however, with high setup complexity and increased cost comes a huge range of benefits compared to the other forms of migrations discussed so far. The advantages include:
Testing the waters: This migration method is perfect for you if you are not ready to fully commit to Office 365 and your email infrastructure is hosted on a minimum of Exchange Server version 2010. This method allows you to assess/review the impact of the move to the cloud on the user experience front. Likewise, you can determine the total cost of ownership (TCO) and see if it is genuinely worth the cost while still maintaining your on-premises infrastructure. Free up space: This method also affords larger enterprises to free up space on the on-premises servers by moving old data (or archives) into the cloud. In addition, the free space can be used to improve the performance of the Exchange organization. It is also worthwhile to note that this is the only form of Office 365 migration that allows you to move mail cross-premise (in both directions) without the help of a third-party tool.
4. Internet Message Access Protocol
The last type of Office 365 migration that we will look at is the Internet Message Access Protocol (IMAP) migration. This migration method allows those businesses that do not have an Exchange-based mail system to move their mailboxes into Office 365. The downside of this method is it only allows you to migrate the content of the ‘inbox’ and any other email folders. It does not allow the migration of contacts and tasks, for example. In addition, for this sort of migration to Office 365, administrators will have to manually provision mailboxes for users as they are not automatically created.
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