How to customize default email for sending mails for case registration for different business units

Hi,

We have several business units and for each business unit there is separate mailbox. 

We are facing an issue when we create a case manually for a specific business unit then email should get triggered to the contact with the Business unit specific email but every time we create a case regardless of the business unit , the mail is triggered from a single mailbox.

How can we resolve this problem?