Today, CPU horsepower is significantly less expensive and is no longer a constraining factor. With that constraint lifted, the primary design goal for Exchange 2019 is for simplicity of scale, hardware utilization, and failure isolation. With Exchange 2019, we reduced the number of server roles to two: the Mailbox and Edge Transport server roles.
The Mailbox server in Exchange 2019 includes all of the server components from the Exchange 2016 Mailbox and Client Access server roles:
Mailbox services include all the traditional server components found in the Exchange 2016 Mailbox server role: the Client Access protocols, Transport service, Mailbox databases, and Unified Messaging. The Mailbox server handles all activity for the active mailboxes on that server.
Client Access services provide authentication, limited redirection, and proxy services. Client Access services don't do any data rendering and offer all the usual client access protocols: HTTP, POP and IMAP, and SMTP.
Along with the new Mailbox role, Exchange 2019 now allows you to proxy traffic from Exchange 2016 to Exchange 2019 in addition to Exchange 2019 to Exchange 2016. This new flexibility gives you more control in how you move to Exchange 2019 without having to worry about deploying enough front-end capacity to service new Exchange 2019 servers.
The Edge Transport role is typically deployed in your perimeter network, outside your internal Active Directory forest, and is designed to minimize the attack surface of your Exchange deployment. By handling all Internet-facing mail flow, it also adds additional layers of message protection and security against viruses and spam, and can apply transport rules to control message flow.