Office 365 - Performance Issues with Shared Mailboxes

When you try to access folders in a secondary Microsoft Exchange Server mailbox that was added to your Microsoft Outlook profile, you may experience random performance problems. 

This problem occurs because Outlook 2010 locally caches (in your Outlook data file [.ost]) all folders to which you have access in the secondary mailbox. This is a change from earlier versions of Outlook. Outlook 2003 does not cache any shared folders, and by default, Outlook 2007 caches only those shared folders that do not contain email (such as Calendar, Contacts, and Tasks).(Microsoft Support)

This is due to the shared mailbox reaching the MAPI session limit, which is around 1000 in Exchange Online. We have found that unchecking the “Download Shared Folders” option on the shared mailbox will help improve performance. While it does run a little slow at first, it does catch up and is a good workaround for now. Otherwise, minimize the number of folders you have in the shared mailbox.

Screenshot of microsoft exchange dialog box

Getting Help

If you need assistance accessing or setting up your Office 365 account, please contact the Technology Support Center by Phone: (909)537-7677, Email:, or visiting us on the Web: Support Center If you have any Office 365 questions, contact the team directly at