Hi Kinan,
I understand that the original mail flow issue has been resolved. Now, the Exchange on-premise mailboxes which had been enabled in Office 365 cannot be moved to Exchange Online.
To troubleshoot this issue, I suggest you perform the following steps:
1. Run the command Remove-MsolUser in Windows Azure Active Directory Module for Windows PowerShell to remove the problematic user from Office 365.
References:
Remove-MsolUser
http://technet.microsoft.com/en-us/library/dn194132.aspx
Manage Windows Azure Active Directory by using Windows PowerShell
http://technet.microsoft.com/en-us/library/jj151815.aspx
2. Make sure the problematic user is moved from Office 365.
3. Run DirSync to sync the user.
4. Try moving the mailbox for the user to Office 365 again.
If any error message is received when performing the steps above, please capture a screenshot for further research.
Thanks,
Mindy Pan