You need to migrate mail from Exchange On-Premise to Office 365.
Note: you should not create any O365 users before running a cutover migration task.
- Log on to your O365 admin panel.
- Switch to the Exchange Admin panel
- Switch to recipients > migration, click the + and click ” Migrate to Exchange Online”
- Select “Cutover migration” and click Next.
- Enter the hostname for your Exchange server and RPC proxy (usually the same machine). Click Next.
- Name the migration batch.
- Select a user or contact who should receive the results of the migration and click New.
- The migration will start immediately; depending on how many users and how much email each has, this could take several hours. Note that users will be created in O365 as mail is migrated in. If a user already exists in O365 for some reason, that migration task will fail and no mail will be imported