Unable to Migrate mailbox to Office 365 - You can't use the domain

When migrating a mailbox from on premises Exchange Server to Office 365, it failed with below error message.

MigrationPermanentException: You can't use the domain because it 's not an accepted domain for your organization.

I followed the below troubleshooting steps to resolve the issue.

1. Log on to Exchange Admin Center (EAC)

2. Click Mail Flow

3. Click Accepted Domains

4. Make note of the domains

5. Log on to Office 365 Admin Center

6. Click Setup

7. Click Domains

8. Ensure the list of domains you noted in on premises EAC is also added (must) and verified (optional) in Office 365

This issue was caused by the user's mailbox having an email address in on premises Exchange server but the domain is not added in Office 365. After adding the missing domain in Office 365 Admin Center, waited for Azure Connect AD sync to complete and then resumed the mailbox migration. The mailbox was migrated successfully.