Besides the primary mailbox Exchange allows access to other mailboxes, which include:

• mailboxes shared with a number of users (group mailboxes)
• mail archive mailboxes assigned to each account (Office 365)
• explicitly shared folders (single folder(s) from some user, which have granted another user view or modify access to them).

The EWS API should provide the necessary information on how to access these folders/mailboxes.

Copied from: https://github.com/KrissN/akonadi-ews/issues/13