Users receive copies of messages after a mailbox is moved to Office 365 dedicated/ITAR (vNext)


After a user's mailbox is moved to Microsoft Office 365 dedicated/ITAR (vNext), they receive copies of message threads in which they select Reply All.


This issue occurs because migrated mailboxes have a new legacyExchangeDN attribute that won't be resolvable locally until the user downloads a new offline address book (OAB).


All Outlook clients that use cached mode will download a new OAB after a move to vNext. This is an expected behavior and will automatically occur within 24 hours. The OAB check that's performed by the client happens at a random interval. Therefore, you may have to direct users to manually force an update if they experience the issue that’s outlined in this article.

To manually download the address book, follow these steps:
  1. Locate Send/Receive, and then select Send/Receive Groups.
  2. Select Download Address Book.
  3. Select Download all changes since last Send/Receive, and then select OK.

Article ID: 3182631 - Last Review: Aug 2, 2016 - Revision: 1