It is a basic email solution. There are two email options, including Exchange, which can be built internally, and Office 365 for cloud-based Microsoft email. The choice depends on the organization. I've worked with both Exchange and Office 365.
The internal email system allows setting up the servers and domain controllers are employed. It facilitates internal communication for the employees. For external email services, other services are needed to be implemented within the same solution. It involves sending external emails by activating another service.