O365 mailbox is not provisioning

Occasional Contributor

Hi folks,

We have on-premises Active Directory with hosted Exchange server. Mailboxes are stored on-premises, Users are managed on-premises and synced to Office 365 via Azure AD Connect. Configuration is NOT hybrid because of Exchange service provider.

I’m migrating mailbox content from on-premises mailboxes to Exchange online using Bittitan tool.

All users synced successfully, except two.

I noticed that these two users’ mailboxes can’t be synced, because they don’t have cloud mailboxes provisioned.

In the Mail settings of these users I can see message:

This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed.


Anyone knows how can I make these mailboxes provisioned?




5 Replies
If the users are properly licensed for exchange it’s probably due to the msexchmailboxGuid attribute!

Read more here:


Thanks@adam deltinger ,

As this will clear msExchMailboxGuid  for all my users I'll need to re-assign licenses for all of them, right?

What could be other consequences of this?



best response confirmed by Andy_Bond (Occasional Contributor)
I would set this attribute to null via adsi edit on those two users, then do a sync again! The licenses are already assigned right? They need to be in order to create an online mailbox

Thanks @adam deltinger  that worked for me!

I cleared the msExchMailboxGuid for both of mailboxes in ADSI , waited for sync and voila.

Mailbox was provisioned straight away for one account, and for another I had to reapply the Exchange online license.


It works now and Bittitan successfully synced messages for these mailboxes.



@adam deltinger  Do you know the circumstances under which msExchMailboxGuid can become populated?