Yes, you can create a new system mailbox.
This article discusses how to change the System Mailbox for EV 5.
Sometimes changing the system mailbox for a particular service is required. This may be because the system mailbox gets corrupted or deleted.
The System Mailbox is used for all Enterprise Vault services that directly communicate with Exchange Server.
These services include:
Enterprise Vault Archiving Service
Enterprise Vault Public Folder Service
Enterprise Vault Journaling Service
Enterprise Vault Retrieval Service
MORE INFORMATION
Create a new mailbox and user account through Active Directory Users and Computers. Attempt to log into the mailbox through an Outlook Client.
Note: You may need to force domain replication in Active Directory Sites and Services or run the Recipient Update Service from the Exchange System Manager to make sure the account has been stamped with an e-mail address.
Once you have successfully logged in, stop all Enterprise Vault services.
Close out of the Enterprise Vault Administration Console.
Within the SQL Server Enterprise Manager, expand YourServerName, Databases, EnterpriseVaultDirectory. Select Tables.
Right click the Service Entry Table that contains the system mailbox that you want to change. (e.g. - ArchivingServiceEntry)
Note: If the same system mailbox is used for all services, then you will need to change it on all Service Entry tables.
Select Open Tables\Return All Rows
Change the System Mailbox value to the name of the new mailbox that you just created.