The Exchange Admin Center in Office 365 is used to migrate data from a hosted and on-premises Exchange environment or an IMAP environment through the creation and management of migration batches. Migration batches are specific requests to migrate all mailboxes or a subset of mailboxes from a remote mailbox source.

Staged migration
Staged migration enables migration from an on-premises Exchange 2003 or Exchange 2007 deployment. The main difference in this kind of migration is that the migration is targeted at customers who want to establish a permanent mail coexistence with Exchange Online and their on-premises environments. This requires that customers deploy directory synchronization when they deploy Exchange Online. This option is available for customers who have Enterprise Exchange licenses only. IT admins must provide a list of users to migrate in each batch by using a comma-separated values (CSV) file.

Cutover migration
The cutover migration option is for customers who want to migrate their whole environment at one time. This migration is limited to less than 2,000 mailboxes and is targeted at business customers who are running Exchange 2003 or a later version in their environment. Specifically, this Exchange migration to Office 365 supports users who are using the Microsoft Exchange Autodiscover service against Exchange 2007 (or later-version) environments to determine the mailboxes that are available for migration. However, if you're running Exchange 2003 in your on-premises environment, you can manually enter the remote procedure call (RPC) proxy addresses to access the Exchange mailboxes for migration. This option is available to Office 365 Enterprise customers and Office 365 Business customers.

IMAP migration
IMAP migration is targeted at customers who may be using non-Exchange-based mail systems in their current hosted environment or have access to the data only by using the IMAP 4 protocol. The Office 365 migration engine uses the information that's provided by IT admins in the EAC to connect to IMAP mailboxes and download mailbox data. IT admins must provide a list of users to migrate to each batch by using a CSV file.

I hope this information helps!
Regards,
Lewis
Tech-consultant
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