One of the largest and oldest consumer email services on the planet that is still widely used today.
I spent many years at Microsoft, primarily as a UX Program Manager at Outlook.com. A big part of my work was being the product owner for compose during and after the transition from Hotmail to the new "Modern" design. This involved designing the contact picker, the full and inline versions of Compose, and modernizing the file attachments experience.
I've also worked extensively on refining mail management features and human factors work such as keyboard shortcuts and selection.
As a program manager, my process starts with understanding user workflow and scenarios, devising ways to solve it and iterating with design and user research to get the experience just right. A trick in my bag that I often use is building interactive prototypes and bringing them in front of users to validate features that we are working on. There is truly nothing that tells you more about the success of a design than to observe another person use it.