There are three main migration approaches used to get existing mailboxes into Office 365:
- Cutover Exchange migration - used when the desired outcome is to have the entire on-premise Exchange moved into the cloud.
- Staged Exchange migration - used when you want to move groups of mailboxes over time and possibly keep some mailboxes on-premise (also called a "Hybrid Deployment").
- IMAP e-mail migration - used when migration from an IMAP-based messaging system.
- Google Apps/Gmail
- IBM Lotus Notes
- Microsoft Exchange
- Novell GroupWise
- Office 365 (e.g. moving from a P1 plan to an E-plan)
- VMWare Zimbra
The cost will vary depending on the type of migration you choose, see the MigrationWiz site for more information.
There is also a less commonly used method called a "file export/import migration" where e-mails are exported to a file (e.g. a PST file) and then imported into the Office 365 mailbox. This method isn't recommended because it can cause issues with replying to e-mails due to addressing information being stored in the e-mail headers (not seen by users) that is not corrected by this method. The tools used to support the four methods above fix this issue in the e-mails they migrate.