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Methods for Migrating to Office 365

This article is an excerpt from the book “Office 365 for Exchange Professionals”.

Office 365 supports a variety of migration methods that customers can use. The choice of migration method can be influenced by a wide range of factors such as the number of objects involved in the migration, the amount of data to be moved to Office 365, the version of Exchange Server (if any) running on-premises, long-term migration or co-existence requirements, whether the organization uses non-Exchange email servers, and even the budget available to spend on the migration project.

The migration methods that are available are:

  • Cutover migration
  • Staged migration
  • Hybrid configuration
  • IMAP migration
  • Third party migration tools

The best place to start is with the business requirements of the migration project. The business requirements should include factors such as the need to complete the migration by a particular date, whether a back-out option for the migration needs to be included, or whether some email resources will remain on-premises. As you will see when you read through this chapter, each migration method has different benefits and constraints, and they may not all suit the business requirements of the project.

The next items to consider are the technical requirements, as they will often quickly eliminate some of the migration methods and allow the organization to zero in on the approaches that are actually possible for them to use. The diagram below provides an example of the decision making process you can work through based on your technical requirements to understand the available migration methods for your scenario.

Figure 3-1 Deciding what migration method to use

The decision making process begins by asking whether the on-premises environment runs Exchange Server 2003 or 2007. For those environments the next decision point is whether there are more than 2000 mailboxes. Organizations with fewer than 2000 mailboxes are supported for cutover, staged and hybrid migrations, whereas more than 2000 mailboxes are only supported for staged or hybrid migrations.

Real World: Although 2000 mailboxes is the threshold specified by Microsoft in terms of support for cutover migrations, it doesn’t mean that organizations with up to 2000 mailboxes should only consider a cutover migration. The logistics involved in handling an outage for a large number of users, as well as the desk-side support needed to assist with reconfiguring Outlook profiles and mobile devices after the cutover, may simply make a cutover migration too risky and complex for the organization. In fact, many experienced Office 365 consultants consider the practical limits of both the cutover and staged migration methods to be more like 150 mailboxes. Organizations larger than 150 mailboxes should give strong consideration to using a hybrid migration instead of a cutover or staged migration.

The 2000 mailbox limit doesn’t mean that organizations with less than 2000 mailboxes should automatically choose a cutover migration. For example, if the organization wants to migrate their users in smaller batches instead of one big batch then a cutover migration would not be suitable.

When cutover is either not possible or not desirable for an Exchange Server 2003/2007 organization the remaining options are staged and hybrid migrations. For an Exchange Server 2003 environment a migration to Exchange Server 2010 needs to be completed first. For an Exchange Server 2007 environment at least one Exchange Server 2010 or 2013 server must be installed in the organization to provide the hybrid functionality. Both options require directory synchronization to be implemented. Without directory synchronization, your migration options are limited to third party tools.

An organization running Exchange 2007 or higher can choose to take advantage of the free “Hybrid Edition” license available from Microsoft. This allows an Exchange Server 2013 or Exchange Server 2010 SP3 server to be deployed in the organization to host a hybrid connection with Office 365. The Hybrid Edition license can’t be used for a server that hosts mailboxes, but the server can be used during the migration to Office 365 and retained afterwards for managing the Exchange attributes of the on-premises Active Directory objects, and can also be used as an SMTP relay server for applications or devices on the corporate network.

If the implementation of a Hybrid Edition server is not possible, for example due to server capacity constraints, then a staged migration is the way forward.

The staged migration method is not available for organizations that run Exchange Server 2010 or 2013. The same 2000 mailbox support limit exists for cutover migrations, so smaller Exchange Server 2010/2013 environments can still choose to perform a cutover migration. However as we’ve already discussed, large cutover migration projects can be logistically very difficult to perform.

Given that Exchange Server 2010 and 2013 are both capable of hybrid configuration with Office 365 you should give strong consideration to hybrid instead of staged or cutover. Although this is the most complex of all of the migration options, it also delivers the best user experience. Hybrid configurations allow the on-premises Exchange organization and Office 365 to function as though they are the same environment with seamless mail flow, a shared address book and calendar free/busy federation. In fact, most users would not even be aware that they are working in a hybrid configuration with mailboxes deployed in both on-premises and Office 365. A hybrid configuration is also the only option that allows mailboxes to be off-boarded from Office 365 to Exchange on-premises without using third party migration tools.

Hybrid configurations rely on directory synchronization. If for some reason the organization can’t implement directory synchronization then the choices are limited to third party migration tools.

Finally, businesses using non-Exchange email platforms can’t use the cutover, staged or hybrid options. For those businesses Microsoft provides an IMAP migration option for moving mailboxes to Office 365, or alternatively a third party migration tool can be used.

As you can see, the decision making process requires careful consideration even just for the technical requirements. Beyond that there are also other factors such as whether the migration project will be handled in-house or by an external consultant, whether extra training is required for IT staff to understand new features such as hybrid configurations, and whether funding is available to pay for third party migration tools if native migration options can’t be used.

Note: Before you finalise your decision on which migration method to use it is strongly recommended that you read through the example migrations in this book from start to finish so that you can learn about any risks or timing issues that you need to be prepared for. Do not start a migration before you have read through the process from start to finish at least once. You should also consider creating a test environment and signing up for a separate Office 365 trial tenant so that you can perform a hands-on test run of your chosen migration method.

You can learn about migrating to Office 365, and a whole lot more, in “Office 365 for Exchange Professionals”.

Paul is a Microsoft MVP for Office Servers and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul is a co-author of Office 365 for IT Pros and several other books, and is also a Pluralsight author.
Category: Exchange Server


  1. mike says:

    This question has nothing to do with Exchange, but do you know if it is possible to change the Sharepoint Online URLs to a different domain name (which is already registered in the portal) other than tenantname.sharepoint.com – like mydomain.sharepoint.com – Plan E3. I do not see any options in the Sharepoint Admin Portal. Thanks!

  2. dave hood says:

    i’ve always wondered who has managed to “touch” 2000 desktops over a weekend to update the outlook profiles to 365 when doing a cutover.

    wtf do MS get these figures? 2000…utter bollox tbh.

    • Joe says:

      When doing a Hybrid migration it sees it and updates the client appropriately. Mobiles do need updated configuration so if you stage your communications to your users correctly it shouldn’t be an issue to do that. I’ve done that and more using this method.

    • 2000 people all needing a desktop reconfig on Monday morning, could be quite a challenge yeah. No doubt the right approach to scripting and automating would relieve a lot of the pressure though.

      Also consider that there are orgs out there where large numbers of the users are BYOD, or casual/part-time workers, or even ppl who only use OWA to access email. All of that can reduce the burden quite a lot.

  3. Andrew Gitzen says:

    We are in the process of planning our migration to 365, licensed for 5000 E3 accounts.
    There is a lot of planning if you want to do more than just 365 in the end.
    Enterprise mobility with Azure AD premium were thrown in purchase as well.
    To make things fun, we are a holding company with 40+ email domains.
    Book already pre-ordered.

  4. David Sampson says:

    Hi Paul,

    Great article, just one clarification if possible. You say “Organizations with fewer than 2000 mailboxes are supported for cutover, staged and hybrid migrations, whereas more than 2000 mailboxes are only supported for staged or hybrid migrations.” but the flow chart and indeed Microsoft’s articles on the staged migration method suggest staged is only an option if you have over 2000 mailboxes. Which was is correct? Can staged be used for less than 2000 mailboxes also?


  5. Mindaugas says:

    I have a question for this scenario:
    Customer uses Exchange 2010 server which is hosted by some Cloud provider and needs to migrate to Office 365 and use Hybrid configuration.
    Does this customer can apply for free Exchange Hybrid Edition license?

  6. Gordon says:

    I am trying to find some information regarding Ex2010 to 365. We currently use a netscaler in front of our Exchange Server for SSL-Offloading. Because of this we do not have a third party certificate on the server. I am wondering about the best way to do the migration. Can we do it through the netscaler or do I need to install a certificate and by-pass the netscaler.

    • The Netscaler (or other load balancers) can be used in Hybrid. Or you can install another certificate and configure the Hybrid connectivity to go straight to Exchange. It’s really up to you.

  7. Terry says:

    Hi Paul,

    My environment is a Windows Small Business Server 2011 with Exchange 2010 SP3. I would think the best conversion senario would be a hybrid. Because of keeping the active directory log on. If after the conversion our Small Business Server would go down, would we loose all emails while it was down? Or would they still be forward to the Office 365 users? Would users just not be able to og on to Office 365?

  8. Jay Jorgenson says:

    I really love the concept of Office 365 but I can definitely see how it would be complicated to migrate all of your information to a new system. You may think that it is easy but it’s not. I have a buddy who is a migration expert for the office 365 and he helped me with my migration needs.

  9. Chas says:

    Hi Paul,

    I have a question re: Exchange 2007 migration to Exchange Online with an Exchange 2013 hybrid server.

    The URL namespace setup described in the Exchange deployment assistant appears to differ from the traditional exchange 2007-exchange 2013 migration scenario.

    Deployment assistant outlines:
    – autodiscover internally/externally points to the Exchange 2013 server
    – Exchange 2013 has a new namespace e.g. hybrid.domainname.com, Exchange 2007 maintains its existing namespace
    – new OWA cname e.g. owa.domainname.com can be created to point all requests to the Exchange 2013 server which will handle redirection to either Exchange 2007 or Exchange Online.

    Can you confirm if the above all that is required to ensure a successful hybrid deployment/migration with an Exchange 2007/2013 scenario?

    Thank you for your time

    • Generally speaking the deployment assistant gives correct advice. But without knowing the specifics of your migration scenario or what info the ExDA has actually advised you based on your inputs I can’t really give you more than that.

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