Help Desk using outlook IMAP – Best way to manage

We currently have a base of 4 customers and use microsoft outlook to manage a shared I map inbox between 6 users.

I am looking for suggestions to effectively manage in house e-mails as well as external e-mails coming into the imap inbox for staff to action.

We have test director as a logging tool and so a call gets raised in one location.

The problem we are having is how to manage the inbox as the team grows larger and volumes of e-mails increase?

Thanks