Action: Create the list of mailboxes to migrate You create a migration file that contains a list of mailboxes to migrate ...

Action: Create the list of mailboxes to migrate  You create a migration file that contains a list of mailboxes to migrate to Office 365. You can create the list as a CSV file using Excel or another program. You don't have to migrate all mailboxes at once. You can do them in batches at your convenience. You can include up to 50,000 mailboxes (one row for each user) in your migration file, which can be as large as 10 MB. Learn more at CSV files for IMAP migration batches.