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.
Article ID: 3163384 - Last Review: May 13, 2016 - Revision: 1