After your mailbox is moved to Microsoft Office 365 Dedicated/ITAR (vNext), you experience one of the following issues:
A user who has a mailbox in legacy dedicated is listed in the address book together with an icon that represents an external contact.
When you try to add another mailbox in Microsoft Outlook, you receive the following error message:
When you try to add a delegate or folder/calendar permissions in Outlook, the user is listed with a red slash over the icon, and then you receive an error message similar to the following:
All mail user objects that represent on-premises mailboxes should have a RecipientDisplayType object that permits them to be added as an additional mailbox and delegate. These objects are considered "ACLable." (The object can be added to an Access Control List).
For an object to be synchronized to vNext as an ACLable object, the msExchRecipientDisplayType value for the user object in the on-premises AD must have a value of 0 (this will translate to -1073741818, an ACLableMailboxUser, in vNext). This indicates that the object is equal to a MailboxUser and should be an ACLable object in Exchange vNext. This allows vNext mailbox users to add delegates and to add more mailboxes in Outlook for users who have mailboxes in the legacy dedicated environment.
Note Room mailboxes and room lists should not be set to a value of 0 like users and shared mailboxes. For the resources to function successfully during coexistence, they have special values.
You should contact your organization's Identity/Active Directory team to request that the msExchRecipientDisplayType attribute be updated. And after the change is made, you will need to allow time for the change to synchronize to Azure AD and Exchange vNext.