I havenвЂ™t moved any of the system mailboxes yet so all the system mailboxes are in their default locations and need to be moved. You mentioned above that you deleted a mailbox database in which the System Attendant mailbox located, but in Exchange 2013, there is no the System Attendant mailbox and the Microsoft Exchange System Attendant service any more, but the object is still present in the configuration partition. On a system using default settings, all mailboxes are assigned to Default Location, whose auto attendant mailbox is 991. In my test environment, I have Exchange 2010, 20 installed. The Attendant option will transfer the call to the attendant mailbox associated with the location to which the current mailbox is assigned. The number of system mailboxes and where they live will depend on what versions of exchange you are currently running and if you have moved the system mailboxes previously. First, we should find all the system mailboxes and were they are located. Luckily, moving the system mailboxes is pretty simple and can be done with a few PowerShell commands. Every Exchange server has its own System Attendant mailbox, and it can't be moved to another server the mailbox is is automatically created on the default first mailbox database when a server is installed if that database is later deleted, the mailbox may disappear and not get created again on another database. If this step is missed or forgotten it will cause some headaches as most things will appear to be functioning correctly, but you will start seeing some pretty strange issues like not being able to start an eDiscovery search. One task that needs to be completed for both the migration too and coexistence with Exchange 2016 is moving all the system mailboxes from Exchange 20 to Exchange 2016. If you are planning on upgrading to Exchange 2016 then you will at some point be running in a coexistence model with Exchange 2010 or Exchange 2013.

System attendant mailbox