There are a number of ways to distribute emails among collaborators:
Customer portfolio approach: Each employee is responsible for a customer portfolio. They deal with the emails sent by these customers, whatever the subject of the email.
Domain approach: Emails are divided up according to each person's area of expertise. The company is divided into sectors (customer service, product, etc.).
Mixed approach: Each sales rep is responsible for transactional emails for the customers in his or her portfolio. Other emails are distributed by domain.