“This user’s on-premises mailbox has not been migrated to Exchange Online. The Exchange Online mailbox will be available once migration is completed”
This only happens for users that have been synced using DirSync as they are mail enabled with on premises exchange. I do not however want to migrate mailboxes and would like to start a fresh.
To fix this, one should exclude the attribute ‘
msExchMailboxGuid‘ on the Azure Active Directory Connect Tool (DirSync).
-Disable Synchronization from the Office 365 Portal
-Delete all synched users (marked as cloud now)
-Remove these mailboxes from the Office 365 Recycle Bin
-Edit the MIIS so the MSExchangeMailboxGUID does not sync
-Enable Sync again