After a mailbox is moved to Microsoft Exchange Server 2013, you can’t use the Open Additional Mailbox option in Outlook Web App (OWA) if the mailbox is using ANSI mode in Exchange Server 2010.
This issue occurs because Windows Integrated Authentication (WIA) is required for redirection between Exchange Server 2010 and Exchange Server 2013 Exchange sites. WIA is disabled by design in Exchange Server 2010 and Exchange Server 2013 ANSI mode for improved security.
To fix this issue, access the ANSI mode Exchange Server 2010 mailbox by using the explicit OWA URL (for example, https://email@example.com).
Article ID: 3117471 - Last Review: 16 Nov 2015 - Revision: 1