Posey's Tips & Tricks
Planning a Cutover Migration: Who Should Do It and When?
Cutover migrations can greatly simplify the process of migrating from Exchange Server to Exchange Online -- once you clear a few notable hurdles.
Although the process of migrating from an on-premises Exchange Server deployment to Exchange Online has a reputation for being complicated, it does not necessarily have to be.
Microsoft allows for a greatly simplified migration type, known as a "cutover migration." As you may have guessed, however, a cutover migration is not appropriate for every organization.
Before I discuss the details of a cutover migration, you might be curious as to what a cutover migration is. The basic idea is that you can migrate all of your mailboxes all at once. This mitigates the need for complicated coexistence scenarios, as are required when users' mailboxes are migrated in batches.
There are quite a few considerations that must be taken into account before attempting to perform a cutover migration, and not every organization is a good fit for such a migration. Organization size is usually the determining factor for whether or not a cutover migration can be used. If you can't imagine being able to migrate all of your Exchange mailboxes in a single weekend, then it's a safe bet that your organization is not a good candidate for a cutover migration.
Of course, Microsoft does provide guidelines that are less subjective than the one that I just gave you. In order to perform a cutover migration, your organization must have fewer than 2,000 mailboxes. In spite of this limit, however, Microsoft recommends that cutover migrations be reserved for use in organizations with fewer than 150 mailboxes, simply because of the impracticality of trying to migrate thousands of mailboxes in a single step.
The number of mailboxes that need to be migrated is not the only consideration that must be taken into account when assessing the feasibility of a cutover migration. You will also need to consider whether any mailboxes will remain on-premises. Typically, performing a cutover migration implies that every mailbox will be migrated to Exchange Online, and that the on-premises Exchange Server deployment will be decommissioned.
You will also need to consider the version of Exchange Server that you are currently using. Cutover migrations are only supported for Exchange Server 2003 and above. I'm guessing that this requirement won't pose much of a problem since so few people are still running Exchange 2000 or Exchange 5.x.
One more requirement is that in order to perform a cutover migration, you will have to add your current domain name to Office 365. That way, when the migration is complete, you can modify the Mail Exchanger (MX) record on your DNS server to point to Office 365.
Adding a domain name to Office 365 is usually a painless process, in spite of the requirement for you to prove that you actually own the domain (this is usually done by adding a TXT record to your DNS server). Keep in mind, however, that the verification process can take some time to complete, so you will want to add the domain well in advance of your migration. I have personally worked through the domain verification process numerous times and have had it take anywhere from two minutes (which is rare) to a little over a day to complete.
As you can see, the requirements for performing a cutover migration are minimal. There are, however, a few things to think about beyond the basic requirements.
One of the effects of the migration process seems really obvious, and yet I have seen it catch a couple of people off-guard, so I will go ahead and mention it. By migrating mailboxes to Exchange Online, you are moving those mailboxes off of your on-premises Exchange Servers and onto Office 365. As such, you will need a valid Office 365 subscription for every mailbox that you migrate. You will not be able to apply your existing Exchange Server Client Access Licenses to Office 365.
One more thing to think about is that the migration process can be disruptive. Messages received by or sent to your recipients may bounce during the migration process. Even after the migration is complete, there is still a bit of a transition period. Changing the MX record on your DNS server to redirect mail flow to Office 365 does not usually result in an immediate switchover. It can take hours, or even a full day for things to get back to normal. Hence, it is best to perform the migration on the weekend, or at a time when the migration will be minimally disruptive.
About the Author
Brien Posey is a 22-time Microsoft MVP with decades of IT experience. As a freelance writer, Posey has written thousands of articles and contributed to several dozen books on a wide variety of IT topics. Prior to going freelance, Posey was a CIO for a national chain of hospitals and health care facilities. He has also served as a network administrator for some of the country's largest insurance companies and for the Department of Defense at Fort Knox. In addition to his continued work in IT, Posey has spent the last several years actively training as a commercial scientist-astronaut candidate in preparation to fly on a mission to study polar mesospheric clouds from space. You can follow his spaceflight training on his Web site.