There are ways migrating mailboxes from one provider to Office 365. Importing through IMAP-enabled mailbox is one of them. And here is a quick run-down of what the steps are.
First, verify the ownership of the domain and get all the mailboxes ready in Office 365.
Then, prepare a CSV file in the following format:
EmailAddress,UserName,Password [email protected],contoso\terry.admin,password12345
The first row is the header, obviously. And each row under the header represents one user, which is the EmailAddress registered in Office 365, and the IMAP credential that will be used to migrate that user’s mailbox.
Now, create a migration endpoint for the migration. Go to Recipients in Exchange admin center. Switch to migration tab, click the … and choose migration endpoints.
Click + sign, select IMAP and fill in the IMAP server info to create a migration point for import.
Once done, it’s ready to create a new import job to use that newly created migration endpoint. Click the + sign in migration and choose Migrate to Exchange Online.
Select IMAP migration, choose the CSV file prepared earlier, and off you go.
The actual process could take quite some time to finish. Meanwhile, you can check the status of the process at any time by highlighting the job and clicking the View details on the right.
Finally, you can flip the switch and change the MX records to finally redirect the mail to Office 365.