Before you start moving mailboxes you have to make sure that all accepted domains used by mailboxes on-premises are configured in Office 365. This can be tricky, you wouldn’t be the first admin that experience failed migration because of a domain.local email address on an on-premises Mailbox J

Now, when you want to move a mailbox from Exchange on-premises to Exchange Online, navigate again to the Exchange Admin Center, and under recipients select migration. Click the + icon and select migrate to Exchange Online to start the new migration batch wizard.

For the migration type, select Remote move migration which is supported by Exchange 2010 or later.

Click Next to continue. Select the mailboxes you want to migrate to Exchange Online, you can use the people picker feature (click the + icon under Select the users that you want to move) for this, or you can use a CSV file to select the mailboxes you want to move.

Click Next to continue. The migration endpoint (Remote MRS proxy server) we created earlier is automatically selected and shown. Again, click Next to continue.

Import in every migration is how to deal with corrupted items and large items. If needed, increase the Bad item limit and large item limit. You also need to set the Target Delivery domain. The Target Delivery Domain is an SMTP domain stamped to a recipient in the on-premises Exchange environment (that’s moved to Exchange Online) that identifies the accompanying Mailbox in Exchange online. This way mail sent in the on-premises environment is automatically forwarded to the Mailbox in Exchange Online. The SMTP domain used for this is typically <tenantname>.mail.onmicrosoft.com, as shown in the following figure:

Click Next to continue. The last window is about the migration itself. You can set a mailbox where information regarding the move request is sent to, by default this is the account used for logging on at the Admin Center. In our test environment that’s administrator@tenantname.onmicrosoft.com.

By default, the migration batch is automatically started, but it is not finished automatically. When the migration is at approx. 95% it will automatically suspend and keep the source and destination automatically synchronized. Personally I prefer to Manually Complete the batch off-business hours. This will allow enough time for replication in Office 365 and on-premises so users can seamlessly logon to their Mailbox the next morning.

Click New to start the migration. The migration batch will now automatically show up in the Exchange Admin Center:

When you select a migration batch, you can click View Details in the actions pane. This will show more information regarding the migration batch (although not much ;-):

In this window you can click the Download the report for this user in the actions pane. This will open a text file with all detailed information you need for in-depth troubleshooting, or for informational purposes or course:

When the migration is finished it will show as Finalized in the Exchange Admin Center:

You can remove this migration batch, and when you check the Exchange Admin Center the user’s Mailbox will show up in the Mailboxes section:

While her Mailbox on-premises has been converted to a Remote Mailbox:

Licensing a user

When you move a Mailbox from Exchange on-premises to Exchange Online, do not forget to license the user with an appropriate license. If you fail to do so you might run into the scenario where you move the Mailbox, but the user cannot login to the Mailbox in Office 365.

Once the license is assigned to the user account, the user should be able to logon to the new Mailbox.