1. Provisioning runs and has the old mailbox details, it will post a message to the old exchange server, it will fail to connect and then post an event saying that the mailbox doesn't exist on the server and to re run provisioning
2. Provisioning is purely on the AD side, so you target a user, or OU or distribution group or ldap query etc, it then looks up the user and gets the mailbox details, what exchange server it's on etc
Then when the task runs, it loads up all the mailboxes that are assigned to that exchange server
So technically you could have an ldap query that targets anyone on ExchA, but really provisioning targets users, not mailboxes
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