When you move a mailbox to Microsoft Office 365 Dedicated/ITAR (vNext), Outlook downloads a new Offline Address Book (OAB). This causes a heavy, unexpected network load.
When a mailbox is moved from one Exchange forest to another, Outlook downloads a new OAB.
As vNext customers start to migrate users from an on-premises organization or from an earlier service instance of Office 365 Dedicated, each user must download a new copy of the OAB from the vNext forest. Users should do this as soon as they are migrated and log on to their Outlook client for the first time.