A user can't access a mailbox through their Outlook client in Microsoft Office 365 Dedicated/ITAR (vNext).
This issue may occur if the mailbox is set to enforce Azure multi-factor authentication but if all the requirements for this feature aren’t in the vNext tenant.
To fix the issue, disable Azure multi-factor authentication for the mailbox. For information about how to disable Azure Multi-factor authentication in the Office 365 admin center, see Set up multi-factor authentication for Office 365 users.
Article ID: 3187083 - Last Review: Aug 22, 2016 - Revision: 1