Design a email service (microservices layer ) which can handle n number of messages/emails without throttling

Create a JAVA API exposed layers/ abstract layer to handle n number of emails(basically design a email service) .There should be no limitation on the number of emails and the design should be in such a way that there are no resource starvation and should have the ability to handle 100's of applications sending emails. Scalability should not be an issue. Need a Cloud Native Architect/Cloud Architect Looking for a 1. Better quality of design to create our own smtp email service 2. HLD + UML Sequence & component diagrams 3. Robustness of the solution 4. Alignment to 12 factors -cloud native approach 5. Ease to triage in case of issues