DirSync currently pushes up extended attributes into Office 365, that prohibits Office 365 from creating a mailbox if a mailbox is present in the on premises exchange server. This appears to be controlled by the extended attributes in the MS Online Dirsync client.
To access the DirSync gui, you need to load the MIISclient. This client is located in the Program Files\Microsoft Online Directory Sync\SYNCBUS\Synchronization Service\UIShell\miisclient.exe. If you run this tool, and look at the management agent properties, for Source AD, you have the ability to disable certain exchange parameters form being sync.
It appears that if you disable the “msExchMailboxGuid” in the sourceAD (properties), this will enable Office 365 to create the Office365 mailbox when a license is assigned.
Question: What are the other extended extended exchange attributes that need to be blocked in dirsycn that will allow a mailbox creation in Office 365, when a license is assigned to an office 365 account?