When administrators first set up Office 365, many of them migrate email from existing systems. Your organization can migrate email to Office 365 from another system as well. The Office 365 administration user interface can be used for this purpose. You can also do this by using Windows PowerShell to migrate email. Your users are free to import their own email, contacts, and other mailbox information to their newly created Office 365 mailbox.
There are three types of email migrations that can be performed from an Exchange Server:
- Migrate all mailboxes at once (cutover migration). With the help of cutover migration you can migrate the contents of user mailboxes from a source email system to Office 365 all at once. A cutover migration can be performed by using the Setup wizard, or by starting from the Exchange admin center (EAC).
Note: The Setup wizard is limited to migrating 150 mailboxes only. If cutover migration is started from the EAC, you can move up to 2000 mailboxes.
- Migrate mailboxes in batches (staged migration). You can migrate the contents of user mailboxes from a source email system to Office 365 over time using a staged migration. This kind of migration is targeted at users who want to set a long-term mail coexistence with Exchange Online and their on-premises environments.
- Migrate using an integrated Exchange Server and Office 365 environment (hybrid). This type of migration allows you to maintain both on-premises and online mailboxes for your organization and to gradually migrate users and email to Office 365. Hybrid migration is applicable for the following situations:
– You have Exchange 2010 and more than 150-2000 mailboxes.
– You have Exchange 2010 and want to migrate mailboxes in small batches during a specific time frame.
– You have Exchange 2013. There is also an option to migrate email from another IMAP-enabled email system.
The O365 migration engine uses the information that’s provided by IT admins of a non-Exchange-based mail system in the EAC to connect to IMAP mailboxes and download mailbox data. IT admins need to provide a list of users to migrate in a CSV file. When the IMAP migration method is used, only the items in the users’ inbox or other mail folders are migrated. Tasks, contacts and calendar items cannot be migrated with IMAP, but can be migrated by the user.