Office 365 mailbox creation and legacy Exchange attributes

I had this question after viewing Office 365 Hybrid Exchange - Mailbox provisioning.

The link in this solution has expired.

I would like to:
Local AD:  Create user and sync to Office 365 cloud.
Office 365: Apply appropriate license that includes Exchange email.

Problem and possible reasons: Account created but Exchange mailbox not provisioning.  This used to be an old SBS server (upgraded to Win2K12 now).  I think it still has old Exchange attributes,  How do you rectify this?
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Vasil Michev (MVP)Commented:
Well you need to check what objects/attributes you have for this account on-premises. If it's a newly created user, there is no reason why mailbox should not be provisioned for him after syncing it to O365 and assigning a license (assuming no sync errors or similar).

If the user already existed, you will need to clear his Exchange attributes first, in particular the exchange mailbox GUID. Otherwise O365 will "think" the user has an on-premises mailbox and will not provision a new one even if you assign a license.

In any case, you should describe what you've done and what's the current setup in more detail.
vihunterAuthor Commented:
You're absolutely right Vasil.  I thought I would ride the coat tails of the previous question you answered.

Again, you're right; this problem is for existing users.

So I saw a fix that was close to what you said.

1. Remove users from syncing.  (Delete from from O365)
2. Clear attributes through filtering in AAD connect
3. Re-add user.

Do you have a list of exact attributes?  And also if I do remove from syncing, don't the accounts still exist in recycle bin?  Are thise above steps correct?
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Vasil Michev (MVP)Commented:
You can remove the users from the Recycle bin as well:

I wouldnt recommend doing anything with the on-premises account unless you've properly backed up the mailbox first. If that's already done, you can just convert those to mail users, which effectively strips their Exchange attributes.
vihunterAuthor Commented:
There is no other on-premise server anymore.  This was an old SBS server that was upgraded to WIN2K12 and their current Hosted Exchange is at a third party solution with no AD sync.

Now that I am trying to move them over to O365, the lingering Exchange attributes are blocking it from create an O365 mailbox.

How can I remove or clear exchange attributes in bulk?

Thanks for your patience.
Vasil Michev (MVP)Commented:
Well if you dont have any Exchange servers, just go with clearing the Exchange mailbox guid attribute.

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