Mailbox does not exist error when migrating into a Azure Directory Synced user account (4318021)

Using On Demand Migration (ODM) to migrate a user into new mailbox in a hybrid Office 365 environment, the migration fails with the error:

Mailbox does not exist.

Checking the user's Office 365 account shows this error:

This user's on-premises mailbox hasn't been migrated to exchange online

Note: After the Azure Dirsync happens, the Office 365 Mail User object will be unlicensed. Using ODM to migrate the mailbox will license the account and and then fail to find the user's mailbox and stop that user's migration.

Check the on premise Active Directory account for the following Exchange attributes.  Removing the attribute data from the object maybe required.  Then perform a new Azure Directory Sync to have the mailbox created as the on premise server owns the object.

• MsExchMailboxGuid
• MsExchRecipientDisplayType
• MsExchRecipientTypeDetails

Note: More information available here and if needed reach out to Microsoft for assistance.

https://answers.microsoft.com/en-us/msoffice/forum/all/this-users-on-premise-mailbox-hasnt-been-migrated/5735f499-7079-42a4-a5e9-8da275404d09