When you configure ADConnect for Exchange Hybrid you expect that a UserMailbox from on-premise will be represented in Exchange Online as a MailUser. I had an issue where one mailbox on-premise didn’t have a recipient in Exchange Online. I checked the Admin Center and found the user was synced from on-premise Active Directory successfully. When I clicked on the user I saw the following error “Exchange: The execution of cmdlet Enable-MailUser failed.”
I moved the user in Active Directory to an OU that wasn’t in sync with ADConnect. Running a delta sync with ADConnect removed the user from Azure Active Directory. I then went to Deleted Users in Azure Active Directory, found the user and clicked on Delete Permanently.
Finally I moved the user in Active Directory back to the original OU and ran another delta sync with ADConnect. This time when the user was provisioned in Azure Active Directory they are correctly showing as a Mail User and I’m able to migrate the mailbox.