While testing with Exchange 2013 CU9 in our lab environment we utterly destroyed the DAG and after recreating the DAG and the Mailbox databases we found that the Arbitration Mailboxes were in a corrupt state. The accompanying user accounts were still available in Active Directory, but mandatory properties were incorrect. This was clearly visible when running a Get-Mailbox –Arbitration command:
In Exchange 2013 there are regular Mailboxes and there are Shared Mailboxes. The latter are user independent Mailboxes and have common email addresses, for example firstname.lastname@example.org or email@example.com. These Mailboxes don’t have a specific user account but are mostly shared between a number of users.
The interesting part is that you don’t need a license to implement a Shared Mailbox (or a Resource Mailbox for that matter) but the users that access the Shared Mailbox need to be licensed though.
There used to be a 10 GB size limit on Shared and Resource Mailboxes, but this was changed recently (November 2014) and all Shared and Resource mailboxes are now restricted to 50GB, just as regular Mailboxes. You can check this on the Exchange Online Limits page on Technet.